You need to look at it from the perspective of bigger picture and end result, Big Red 1.
End of the day, your idea is simply "better AT capability". Which you have to admit, can be solved by simply buffing the M4's main gun in terms of penetration and/or base damage.
Never mind whether it makes Easy Eight redundant, the idea you propose results in just that. So you wouldn't actually need something superficial like an unnoticeably longer gun barrel that is expressed as a 60-90 munitions upgrade (that takes up time and resources to dom and also becomes a burden to keep alive).
Question then, is whether your idea for 76mm gun upgrade is merely a matter of balance considerations ("more offensive capability"), or just a matter of getting something that just/ also looks it? Because if it simply a mater of M4 needs better pen, then you just increase their pen, you don't need to get convoluted with a whole side-tech upgrading that you'd need to do with every tank. We're not talking pintle MGs that only deal against infantry and anti-air here, we're talking a drastic change to the main medium tank of this faction to increase its base firepower.
In any case, having it as non-doctrinal also makes Easy Eight less useful of a commander option. You'd have to make E8 much more powerful (which is unreasonable) to make it a viable alternative to spamming M4s, and also means every USF player would just spam M4s than risk making Jacksons.
So what would happen is thus:
-players don't bother choosing Riflemen Company and just spam M4s
OR
-players do choose Riflemen Company, and never bother with this upgrade ever.
EDIT: While we're on that subject, what exactly is different, stat-wise, with Easy Eight compared to M4? Obviously the 76mm gun, but what else, does it move faster, higher health, less accuracy penalty when firing on the move?
why do usf at ap rounds deflect of front