In nature, a lack of genetic diversity in a species isn't typically much of a problem until there's an extinction event, and then it can be a big problem.

You might not feel the downsides of having a huge percentage of the internet going through a single provider until it's too late to change it.

I actually quite like Cloudflare, and I don't think they're purposely doing anything nefarious (just following incentives like everyone), but I think caution is warranted.