I would offer that there are clear distinctions between an infantry fighting vehicle, capable of killing comparable vehicles with its main cannon or ancillary ATGM, and an infantry support vehicle, capable of rendering support in any sort of built-up environment.

One needs to consider how the US Army has organized and equipped itself in the past to accomplish those two discrete tasks, before asking the question about caliber size for any main cannon.

When the Bradley was developed, side saddle TOW launchers were emplaced in the design, but the unit organizations still M1 tanks mixed in to provide the wall-busting support required, and the development of 120mm ammunition has followed an arc aimed at penetrating and reducing heavy cover.

As the Army transitioned to the Stryker and its remote weapon system of no larger than 40mm AGL or .50 HMG, it bought the 105mm Mobile Gun System variant to provide infantry support within those formations, since the M1's 120mm main gun was lost.

The US has never had a one-design-fits-all mindset when it comes to mechanized infantry operations, like the Russians have through the BMP-1 through -3 series of IFVs, and there is some inherent flexibility (and for sure some weaknesses) from that approach.

It seems the US Army stuck with what was in its training and ammunition system not so much out of a desire to avoid additional cost, but because that system and combination of capabilities has proven capable enough to meet the infantry's needs. If it were found woefully wanting, there would for sure be pressure to adjust and implement new systems, but the current crop of tools is generally adequate and the US Army mindset has recently trended towards C4ISR upgrades before ordnance changes.

I would guess that we won't see a main cannon for another couple of generations.