Problem with hotter powders, is that the wear on weapons that wasnt made for them is much higher.
It is not so much the powder as the muzzle velocity... very high muzzle velocity rounds in the 1,000m/s plus range are very hard on barrels.
I am not suggesting increasing muzzle velocity for the sake of it... the calls you mention to move from 5.56 to 7.62 are largely based on the fact that the smaller calibres have bullets that are simply too light to be effective at longer ranges.
The purpose of my suggestion regarding the new powders in the 5.45mm weapons is not to greatly increase the muzzle velocity... 950m/s is the normal muzzle velocity of 5.45mm ammo in the RPK-74 and it has no problems with barrel wear.
I am suggesting the new powder to allow an increase in bullet weights that the 6.8 cal weapons were supposed to offer with more speed than the 7.62 x 39mm offers to get the combination of flat shooting and short bullet flight time, with greater bullet weight leading to it retaining energy over greater distances and also for increased penetration and effect on the target.
Using standard powder in the 5.45mm calibre case with a larger bullet greatly effects performance.
An 80 grain bullet leaves so little room in the case for the standard powder the muzzle velocity is actually subsonic.
Or to put it another way the heaviest bullet they could cram into the case for a subsonic load was only 80 grain.
This underwater projectile takes up even more internal space leaving even less space for powder... to actually make it effective they had to develop a whole new type of powder.
If they are producing that powder anyway they might as well make some special rounds too.
AFAIK the Russian MIC have been tasked with making Russian small arms 2.5-3 times more effective... this means better range, better accuracy, and presumably better lethality.
A change in calibre is enormously expensive... they already dabbled with a 6 x 49mm round that supposedly had a 120 grain bullet travelling at a muzzle velocity of something like 1,200m/s which sounds pretty hot to me. It was to replace the 7.62 x 54mm round and there was a machinegun to replace the PKM that fired it and a sniper rifle that looked like an SVD too.
New more potent ammo will give them an excuse to work their way through their store of weapons fairly rapidly or to update their armoury.
I agree a new from scratch weapon family that is being developed needs a new round, though they haven't had any problems with the 5.45 in terms of lethality AFAIK... it is the 5.56 that is failing.
As i have read, and experienced, most users of 5.56 is going for the 7.62 or a new 6.8. In that hindsight, and with almost everyone using bodyarmor, i think it will be a mistake to produce a ultramodern new assaultrifle with 40 year old ammo.
7.62 x 51 is actually older than 5.56 and it is the best current solution... will get out my SLR and clean it for a bit and chuckle over that tonight...