The US Army wants a 50mm cannon on its future IFV...what does that mean for the USMC?

You've read the story about General Dynamics taking the lead for the US Army's future IFV.  What you missed was this.  via Breaking Defense.

AUSA: General Dynamics looks like the early favorite to replace the Army’s 1980s-vintage M2 Bradley troop carrier. That’s my personal assessment after talking at length to officers and contractors at last week’s Association of the US Army conference, where months of uncertainty finally gave way to some real clarity about both what the Army wants and what industry can offer.

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 Only the Griffin actually has a 50mm installed right now, however. The others currently have 35mm cannon. It’s also the only vehicle that can point its gun almost straight up, at an 85 degree angle, to hit rooftop targets in urban combat, something the Army has worried about extensively. Details like this suggest that General Dynamics has been listening more closely to the Army than its competitors. In fact, even where the Griffin III underperforms its competitors, most notably by carrying fewer infantry, it does so in areas where the Army is willing to make tradeoffs.

Story here.

I probably could have done a better job of cutting up this story to illustrate my point but I think you get the general idea.

The US Army wants a 50mm gun on their ride.

I'm shocked. 

This is the first I heard of that.

The discussion has been between the 30/35/40.  Now we see they're by-passing all that and heading straight to a 50mm cannon?

My question is this.

Does this mean (and I think it does) that even APC/IFV are becoming armored enough to withstand 30mm cannon fire?

The implications of that are staggering.

We're talking about a world where its becoming at least a THEORETICAL possibility of IFVs engaging MBTs at least from the sides and rear with their main gun and getting penetrating hits.  I'm positive that once the geeks get to play with the round that we will see that as automatic pens after a few years of it being in service.

Am I reading this wrong or is the US Army (following Russia's lead...remember they came out with that 57mm) changing the armored warfare game?  And if they are then what are the implications for the USMC?  We're dependent on ATGMs for our infantry when engaging armor, but with APS coming online and with 50 or 57mm becoming the new norm, how do we defend ourselves if Marine Air is off on another mission?