Well... not really.
KV-2 was a super tank that was supposed to support attacks on heavy structures that a standard 76.2mm gun would do little to.
It really lacked the mobility to operate with infantry or armour in open country... in many ways the KV-2 was a siege engine... its role being to sit and fire and reduce heavy concrete fortifications... it would be the sort of vehicle you might have used to take on the Maginot line... they would have selected an area where the defence guns couldn't reach and it would just sit there and pound its way through.
In built up area fighting it could have been a powerful weapon to reduce buildings and anti tank structures to scrap, but in the mobile fluid tank warfare it ended up being used in it was a bit of a failure. If they had had enough tanks to pick and choose a fight they could have kept them aside and waited for urban combat or a siege, or when the Germans fortified a line it would be handy to go in there and smash everything with its enormous gun.
About the only tank destroyers they had were the SU-85 and SU-100 both armed with very powerful and very high velocity weapons that were very effective against any contemporary German armour... I believe the Soviet nickname for the Su-100 was translated into "f'ing end to everything". It was held in the greatest respect by the German forces too who feared its power.
Apart from those two vehicles and their ISU equivalents all the rest were basically direct fire artillery used to support tank and motor infantry operations.
The BMPT concept is not some sort of super tank, it is basically to compliment the tank... to engage the targets that threaten tanks that tanks are not good at fighting because their guns only elevate to about 20 degrees up down to perhaps 5 degrees down... it is a very limited range of elevation.
Late model BMPs have the firepower to do that, but they lack the armour to follow tanks everywhere.
With the new brigade structure it is all together possible that the BMPT has become redundant because the armata based IFV will have the firepower needed for the role so an armata IFV without the troops and extra ammo for its main weapons could be the solution.
Remember the model with the 120mm rifled gun/mortar is just a model.
It could offer another alternative in that the IFV is optimised for fighting IFVs... and the BMPT doesn't really need that because the tanks they operate with and the IFVs they operate with should be able to handle any armoured threat at much longer ranges.
This suggests that taking the gun/mortar of support vehicles and a 23mm gatling gun and a 40mm grenade launcher would offer the HE firepower in close that would be needed to suppress and enemy assault or attack on an armoured force.
I would love to see such a vehicle... that 23mm gun would be devastating... twice the rate of fire of the US Vulcan 20mm cannon with a much heavier HE projectile... it would be truly devastating... even in short bursts.
A quarter second burst would send 50 shells downrange, and though the muzzle velocity is only about 700m/s with HE shells it is not velocity that matters... a tight cluster of 23mm projectiles exploding would be devastating.
I quite agree with you. BMPT doesn't need 120 mm mortar or any other big gun. It works with tanks, so why place 120 mm mortar on it, when 125 mm HE shell from tank will do the same job? Big guns only take valuable space for ammo, increase weight and decrease elevation.
A 125mm HE shell is a relatively high velocity round from a gun that can't elevate beyond about 20-25 degrees. It can't elevate to hit targets above it and targets behind cover are totally safe... it is the same reason the BMP-3 has a medium pressure 100mm rifled gun... it packs a lot of punch and can elevate to lob shells over barriers. The difference is that the 120mm gun/mortar is already in service and use and can use a range of ammo types. It can carry a heavier round than the 100mm 2A70 gun of the BMP-3, and comes as standard in a mount that offers very high elevation options because it doesn't need a long recoil stroke.
30mm gun on BMPT is primary weapon, not secondary. Secondary is ATAKA ATGM. When choosing guns for BMPT, you have to know two things. Bigger caliber gun have too low rate of fire, gatling gun have too high rate of fire and you will be too quickly out of ammo.
IMHO the best compromise is to have both... the 23mm gatling has a very high rate of fire, but would not be used continuously to hose down an area. The 23 x 115mm round is very small compared with even the 30mm round so a lot more 23mm cannon ammo could be carried in a given volume.
As a weapon it is actually lighter than either of the two 2A42 cannon in the old BMPT and as it is gas powered it does not need an external electric motor to power it. Using it for very short bursts would make it an effective weapon.
The Shilka has already been used in the BMPT role, and its only real problem was that had relatively thin armour and that it needed to carry more ammo... the latter problem was solve by ripping out the electronics and loading more ammo.
The 23mm gatling uses the same projectile as the Shilkas gun, but with the much smaller propellent case the projectile just takes a little longer to get there.
The target really would not know the difference except that with a Shilka there are 4,000 projectiles per minute landing around them, but with a single 23mm gatling gun, which weighs 73kgs compared to the 2A42 which weighs 115kgs each, would be firing the same projectile at up to 12,000 rpm... 3 times faster... or enabling the same burst 3 times quicker.
We could debate about compromise constructors choose for BMPT with two 2A42 guns and place instead GSh-30-2 or any other gun with similar capabilities, but for my opinion this compromise is good one. Only things I think they have to improve are higher elevation for main guns up to 70° or more and installing day/night sight in its FCS, which could follow guns to the higher elevation. Maybe they could also increase the number of ATGMs from 4 to 8, what is more than enough. Anything else is job for tanks or artillery and TOS behind.
But that ignores the situation... a tank often can't elevate to hit the target in question, while artillery don't operate directly with the tank regiments or infantry regiments... they sit further back to the rear and respond to requests for support. With a BMPT the vehicle is right there with you and is available all the time to hit targets allocated to it. It is in the fight with you and is always available to use.
It is like having all the Machine guns in the rear and having to radio back to get them to engage targets for you... it is not as useful.
The BMPTs armament is a bit like a BMP, except instead of supporting the infantry it carries it supports the armour around it. It will have a range of targets to deal with so it needs a variety of weapons.
Missiles are a stopgap answer... a band aide answer. It is better that the vehicle be fitted with the tools to do the job.
That armata BMPT model is ideal in my opinion, but as a cheaper and lighter alternative I would think replacing the 120mm rifled gun with the 100mm rifled gun of the BMP-3 could be an option to increase ammo load or for commonality reasons, but at the end of the day the 120mm "shells" (I use ""s because the shells for the 120mm gun/mortar are just projectiles with a stick out the rear with propellent bags tied to it like a howitzer round) are compact and very powerful and would be ideal for the job.
I think all the weapons slaved to the turret makes sense because these weapons wont be all used at once... a specific target will require a specific weapon.
I think in COIN type operations the BMPT would actually be more suitable to a "gun platform" type role, and certainly in the convoy protection role (like the US used the Vulcan M163 in Vietnam) it would also be ideal, and for airfield protection it could operate with Pantsir-S1s in case of a ground attack was mounted on an airbase.