Oh boy, you went there. No one is talking about genocide to eliminate diversity, just that there are instances where diversity can be a hindrance.
Once again, those Wikipedia articles are highly recommended reading.
The main example I pointed out is American politics. There is quite a bit of diversity of opinion and that makes it far more difficult to reach a consensus about a particular issue than you'll find in e.g., European and Asian countries, which have far more homogeneous populations (and by extension, far more homogeneous political/social opinion). As a result, the speed with which progressive policies can be implemented in America ...lags... our European brethren, to put it lightly.
I pointed out that you were talking like a Nazi because, in this instance, you were talking like a Nazi. Do I think you secretly harbor a desire to wipe out an ethnicity via genocide? Nope. Do I think that criticizing ethnic diversity as standing in the way of progress sounds suspiciously fascist? Why yes, yes I do!
Identity politics has no doubt hamstrung the American left, but that has had little to do with your cited example of the failure to implement a nationalized healthcare system. Diversity increases viewpoints, more viewpoints invite stronger debate. Does less debate lead to better outcomes, or faster outcomes? Which is preferable?