How do I build a mobile mortar as UKF? Doctrine, to fight against a sandbag? or spend 350 mp on something that is static and an indirect fire magnet. You're right that mortars are the answer to entrenched units, but not all factions have access to them as easily as Ost.
OKW may have to back tech, USF same, SOV same and UKF, well, either doctrine or gamble on 350mp, and if the sandbags on your cut off for example, youll have a hard time finding a place to build one.
Use UC with flamer, you get suppression resistant long range flame to force enemy out of cover.
Less spam because generally there are only 1-2 engineer units around and they're needed for other roles too, so you'd only be able to build sandbags in vital places rather than place them everywhere your 3-4 mainline infantry squads go.
It was a mistake from Relic to add sandbags to mainline infantry (Cons could've gotten away with it). However currently sandbags on mainlines are a big part of their power level and tinkering with that would cause ripple effects that would probably require quite significant adjustments to fix, so that's not really something that could be done at this point.
I respect your point of view, but I believe that sacrificing a squad of infantry for an extra engineer squad that provides you with very little firepower just to be able to build more sandbags would not be worth it, especially that after the first 6 mins of the game countering sandbags is pretty easy anyway.
Additionally, having current unrestricted way of building sandbags makes some poor map designs a bit easier to bear and balance, as all factions can build green cover.