Doesn't basically the whole US use either Comcast, Cox, Charter, AT&T, or Verizon? And isn't the number of people who manually choose a DNS server other than their ISP's negligible? And for DoH, besides Cloudflare, there's also Google, Mullvad, Quad9, and tons more <https://github.com/curl/curl/wiki/DNS-over-HTTPS>.
In theory, sure, but in reality they are more centralized.