The mortars of 82mm successors of the old 2B9 Vasilek (over 600Kg) are of man-portable size since the 1980s. The 2B14 Podnos is of 42 Kg, and the modern 2B24 and 2B25 have 45Kg and 13Kg of weight. This is the artillery of man-portable size I was talking about. It is succeeding in the Russian Armed Forces like the self propelled artillery.
In the other side, in recent years the development of heavier towed artillery seems to have the purpose of developing guns for new self propelled artillery models and/or for export, since there is not a single model since the 80s succeeding with significant number or orders for the Russian Armed Forces. The 2B23 (over 400Kg) maybe the best known case and even has not English Wikipedia or warfare.be articles.
Russia has some advantage here. Other countries have not enough developed their self propelled artillery to think about to replace the towed artillery, but Russia can think about it even in the short term. The movement is predictable since the current heavy towed artillery in service is from the 80s or before, and will not remain forever
Mortars are rarely considered as an artillery, but ultra light infantry support weapon. In Serbian army if you call mortar in front of an officer as "an artillery piece" you will probably do 100 pushups. Mortars are mortars, artillery is an artillery.
Also Vasilek is class of its own, its automatic mortar and it simply cant be replaced with something like 2B14, USSR operated 82mm mortars long before Vasilek came into service and still it found its place. Nona variations and variants are not replacement for Vasilek either, unless it gets phased out completely as idea of automatic mortar, but i am not sure about that still.
Problem with self propelled artillery is that its useless in so many regions, it simply cant go to all the places it might be required. When D30 variants go to storages Russia is out of real light artillery. Nona-K for an example imo could replace D30, but what will fill the gap between Nona and MSTA/Coalition? Towed or even self mobile light howtizer in 152mm is equired, something like SLWH Pegasus for an example, abit weighty but still alot less than MSTA-B and helicopter transportable.
Technologically a mortar is artillery and always will be. It is possible to make alternative classifications by function or by other criteria that are compatible with the technological classifications, Serbian oddities appart.
I was talking about 2B14, 2B24 and 2B25 as successors of the 2B9 in terms of damage done by mortars of the same caliber. Today the concept of fight of the 2B9 Vasilek remains in mortars of bigger caliber, of 120mm.
I'm not sure if the 2B9 Vasilek remains active as towed gun in Russia. I know these mortars have been mounted on MT-LBs making them self propelled (an obvious step when a country can use MT-LBs as tractors for the 2B9). I do not agree about the troubles of movility that you see in the self propelled guns over the towed guns.
With the D-30 being the towed version of the 2S1, the natural replacement for the D-30 would be the 2S (the upgrade of the 2S1).
Actually by the book mortars of caliber less than 120mm are classified as "Infantry mortars" and are NEVER being refered to as an artillery piece. Mortars of 120, 140, 160, 240 and other calibers are often classified as an artillery pieces due to fact they are either placed on wheeled platforms or have carriages. But as long as they are "infantry mobile" they are not artillery pieces.
While self propelled mortar 120mm i would call an artillery piece i would never call 60mm mortar that i can carry in one hand as such. By your logic 30mm automatic grenade launcher is also an artillery piece... however its not, its infantry weapon.
When its about Vasilek yes, its still used as towed piece in VDV and Ukrainians use it also that way alot.
Still you cant replace automatic 81mm mortar with random 120mm mortar, does not make much sense at all unless again as i said you are abandoning automatic mortar as idea totally.
How and why would towed D30 be replaced with modernised already existing Gvozdikas with replaced main weapon? Maybe partially but that is not going to happen in my opinion, 2S34 Chosta is reported to have quite an issues. D30s might partially get replaced by something in class of Nona-K and then maybe they will "borrow" tracked 120mm long self propelled mortar from VDV but still neither of those are real and good 1on1 replacements.
Light towed artillery has to exist no matter what you imagine. Self propelled howtizers have many advantages but also many disadvantages over good old towed pieces.