An assault gun makes ilttle sense when you have UMPK 500/1000/1500/3000kg bombs at your disposal. The heavy direct fire siege artillery only came about during WW2 because airpower outside of dive bombers (which required highly skilled trained pilots) had dogshit accuracy.
But aircraft like the Il-2 and Su-25 essentially flying CAS missions with bombs and rockets and cannon provide the accuracy to take out hard points and problems for the ground forces moving forward.
Systems like the Tulip and the Malka exist because sometimes a bigger heavier shell is what is needed and sometimes you can't wait for air power or it is snowing and they can't find their own arses in the prevailing weather.
The reason the BMP-1 remained in use together with the BMP-2 and that the BMP-3 has a 100mm gun as well as a 30mm cannon is because sometimes a heavy HE shell is useful.
You don't need an assault gun when all the strongpoints have been deleted.
Yes, calls to the men to stand up and walk slowly through no mans land because artillery and machine guns have already taken out the enemy so you just walk slowly and try not to step on a land mine on your way to capturing the enemy trenches full of dead enemy soldiers...
When your men start walking towards the enemy position new machine gun positions and gun positions that remained silent before almost always open up... which is when a few ISU-152s come in handy with their sights set to 6km direct fire from behind the tanks moving forward to obliterate anything that opens up.
The one thing going for assault guns is that it is much cheaper and quicker to manufacture than a proper tank. You can also add more armour for better protection due to weight savings. For these very reasons I would seriously consider re-introducing the assault gun concept of old.
Modern front line tanks have 125mm guns which are not a lot less powerful than the assault guns of old, but I would say that in lieu of building custom designed assault guns you could simply use the communications your front line troops would now enjoy and have them call in laser homing 152mm shells on the targets they would use a 152mm gun on the front line against. Being able to lase the target for direct hits means a Coalition 30km behind the front line could be as accurate or even more accurate than an ISU-152 that is operating 6km behind the front line providing direct fire support.
For real fire power you could have a Tulip or Tulip replacement sitting 5km behind the front line dropping laser guided 130+kg bombs on the enemy... with the precision of a guided weapon such rounds would be devastating.
Assault guns had the advantage that they were cheaper and faster to produce than tanks.
Which is still important today.
A modern gun might have an unmanned gun mount that allows better elevation and a bit more traverse and an autoloader and auto ammo handler racking system where the three crew are just in the front hull as with the T-14 while the middle and rear are engine and ammo with a large box structure.
The point is that an assault gun does not need a high velocity round... a scaled up 203mm round based on the 100mm gun of the BMP-3 could be used to lob a super heavy shell a moderate distance to a nearby target. Perhaps even a 240mm mortar based gun but with a shorter heavier barrel designed to fire shells as well as current mortar bombs...