Fantastic and detailed post.
My $.02 is limited to engagements in Iraq, all of which were in an urban setting (so I discount my own opinion a lot on this)….that said, the question for me is, will the individual soldier not carrying a crew served weapon be expected to suppress the enemy or not? If not, you need to be damn clear that you don't pull a trigger unless you have you have a target in your sight picture. However, if they are going to suppress, going from 210 to 140 rounds is a really bad tradeofff.
I suspect the right answer is more M249 like weapons per platoon, and then more individual riflemen that create effects like squad designated marksmen. In my experience, the individual 11B, 0311, 19D, etc slinging rounds wildly out of an M4 created a lot of noise but little effect, and it happened all of the time. Contrast that with a good 240 gunner or SDM that has a big impact, and more specialization seems like it could be a good thing.