The problem here is that you're losing the in-direct fire support capability, and making a player choose between that and a more effective 360 area of denial unit/emplacement is... well, I won't say it but you can guess the word
A reasonable tradeoff?
I don't see how giving no indirect fire if you go Bofors would be viable.
So you don't really want just indirect fire (if that was the case you would just unlock the mortar pit). No, you want indirect fire AND the Bofors.
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Why are people acting as if having indirect fire is some sort of a God-given right for every faction? Yes, other 4 factions all have 2 separate units capable of indirect fire, but then, those factions also do not get access to the
hands-down best defensive position in the game.
The word of the day is tradeoff. People act as if Bofors is a thing you should definitely have access to in a faction. Well, no, there is nothing like the Bofors in the entire game, so I don't need to somehow justify the tech restrictions and tradeoffs I propose as counterbalance. You need to justify what you would give up to get the access to a Bofors. No other faction has anything that comes even near in 360 degree area denial for infantry, light vehicles, and aircraft alike (just compare it to the Luftwaffe emplacements, a commander ability, and weep).
Oh, the match dictates you need indirect fire? Well what the hell are you doing unlocking and building a static emplacement for short range defense for, then? If I go for triple Ostwinds when the enemy has nothing but tanks, should I get to the balance forums and complain about overpowered Allied factions? Your build choices should be dictated by the match and the circumstances, you don't get to build a Bofors and then expect the game is balanced around your sacrosanct right to have this ultra emplacement no matter what, and just let the enemy think about things like unit composition and react to your build choices.
And with my proposal you can get your AEC too. So, a player going for an AEC and mortar pit would be no worse off than any players currently researching the AEC unlock.
Or, you can
opt for this incredibly sturdy defensive emplacement that the enemy will have to invest significant micro and resources to dislodge, but lose the option of indirect fire in the process. Suddenly a Bofors is not a no-brainer, but a choice.
It's not gonna eradicate sim cities, but you can either have a mortar-wiping machine with a long range and zero micro requirements, or a shorter range defensive position that will kill anything short of a tank, but can now be safely chipped away from a distance by LeIGs, mortars and AT guns without fear of nuclear mortar shells annihilating them.