You’re focusing on CloudFlare, but already I have a choice of a couple of DNS-over-HTTPS providers in my preferences, and I see no reason why it would stick at just two.
You might protest that since that's complicated and takes effort the 'choice' isn't that meaningful, particularly when it comes to our collective privacy. You might also protest that it's easy to mess up and hard apply consistently to all hosts, and that few people would bother.
Exactly. The same applies to "you don't have to use cloudflare" for DOH.
You also could have already used your choice of DOH by running DOH proxy-- and bonus: all software would be protected rather than just the browser. ... so if "choice" was the only criteria, we already had it, and there was no reason to default many millions of people to sending all their DNS requests to a single point of observation.
There's certainly precedence for web browsers doing things ahead of OS/system support because they can do it faster, and because maybe they think their users' (privacy and/or security) needs can't wait for deeper OS/system support.
(Also, for an increasing number of people, a browser isn't just "an app", it is their real OS where they spend the most time.)
There's a long Twitter discussion initiated by Paul Vixie that discusses the problem in more detail:
Can you please cite that "couple" and also how you believe a typical user would ever change the default?
As for users changing this, sure, that’s not going to happen. But I can imagine a world where the default is that your requests are fanned out randomly to a choice of hundreds or thousands of providers, with no clear and complete picture of a single user in the event that a single provider is compromised.
Some readers probably aren’t aware of the fact that even with DNS over TLS to Cloudflare the DSL provider can still collect exact info of which sites one visits. The name of the site is visible in the clear in the TLS communication with the site, independently of DNS.
Firefox nightly (not sure with other version) propose already NextDNS as an alternative in pref. and an easy way to add a custom one. I can see for example big companies requiring their employees to use a custom one.