Replying again as I did some research. It turns out there aren't actually only 13 root name servers, there are almost 2000. Also, the vast majority of queries to the root servers are from badly configured systems that aren't caching results properly or aren't even receiving the results. So running your own caching resolver, assuming it's working correctly, would contribute to the small drop in the ocean that is legitimate usage of the root servers. Presumably the same applies to the next levels up too.

It's a good point, I never really thought about it. In my case I'm reluctant to use my ISP servers because my country tries to practise censorship via DNS, but I also didn't really feel like using something like Google. I will do some research and experimentation with upstream caches like Quad-9 and Cloudflare to see what it's like.

DNS is highly distributed.

I doubt the current infra would have any problem handle the load even if all individual devices had a local resolver.