Play
Guy gets in a building with an mg.
Counterplay
You can flank it, swarm it, flame it, nade it, smoke it, mortar it.
And that's all the things you can do in the first 5 minutes depending on how you decide to respond.
If there is a side of the building with no windows, more power to you head in from that side. But you shouldn't be able to SHOOT THROUGH A FUCKING STONE WALL WITH INFANTRY
So basically you only want to be able to take damage from small arms from units standing right in the arc of your mg, where they will get torn to pieces and suppressed.
That's not practical. There is no swarming or flanking that, any infantry will just get suppressed. Your change would make machine guns in buildings too powerful.