Diversity definitely makes implementing generous social programs more difficult politically. The average working-class voter in any country will be much more supportive of eg. universal healthcare if he feels like the benefits are going to people with backgrounds similar to his own, just perhaps a bit worse off, as opposed to (implied-minority) "welfare queens" living off the fat of the land. The larger and more diverse a population, the less said everyman will feel like his taxes are actually serving people he knows and trusts.

That said, I do think we're moving in an encouraging direction. The percentage of the developed world that's willing to recognize others from different cultures and backgrounds as part of their own society seems to be growing, ever so slowly and fitfully. There will be years and perhaps even decades feel like regressions, but on average it feels like things are getting better.

I can see how that might make it more politically difficult to pass something, but it wouldn't make it harder to come up with a plan that works. A plan that works and can pass is a harder, but unfortunately I was already in the realm of fantasy the moment I started talking about looking at foreign countries to see what worked for them.