I hate CloudFlare as much as the next guy but you can change the DoH server, can't you? I hope Firefox will expose the option in the interface and not in about:config. But we all know how Mozilla are...
you appear to have jumped to step 2 in the masterplan, and completely skipped step 1. It's not even mandatory yet, but when it is, the choice will be between N<10 providers, most of them almost certainly American companies
I can't see into the future, unfortunately, but you're welcome to bookmark the parent comment and set a calendar entry to compare it with reality about once every 6 months. It's possible to speculate, though. Google have been engaged in open war against ISPs for most of the past 10 years, it is in their every interest to commoditize the pipe between the consumer and the datacentre as far as possible. Removing DNS from the link is eliminating another source of risk
For Mozilla, I'm not sure, but they often follow Google's lead, and there is a strong case for Google to go that route
In any case if it ever starts defaulting to on in any browser, it's very likely the others will follow suit, as it's easy to imagine quite a lot of PR around the security benefits of the brave new world