If a general purpose browser can empower hundreds of millions or even billions of regular users with better privacy (and ultimately, security) by making a change that might disrupt a small handful of power users who manually configure this stuff, I say the browser should go for it. The power users are the very people who can, without much effort at all, reconfigure their stuff, or easily find a special purposed browser, so they'll be just fine.
Spock was right, logic clearly dictates that the needs of the many outweigh the needs of the few.
The problem I fear is the needs of the few who are not technology minded, don't want their browser (or in their eyes their internet connection) to stop working because their ISP issued router uses a DNS based captive portal to onboard people (I've seen this used by atleast one major ISP in the UK to on-board devices onto their per-device content filtering system - BT, however I think they rolled back on that after it was caused issues with IOT devices).
However I believe (not read the docs in a while) FireFox works around this by falling back to DNS if an issue with DoH is detected.
EDIT: However I'm still on the fence if it should be a browser decision. Yes browsers move more quickly then OS & ISP changes and can make things better for the masses quickly, but i'm also wary of those changes screwing up the avg person, the people like my mother who can just about order things online via her ipad but thats about it, if she accidentally lowers the screen brightness of her ipad I soon get a call about it. Its for those kind of people I don't like the idea of a browser messing around with a connection in unknown network conditions.
> If a general purpose browser can empower hundreds of millions or even billions of regular users with better privacy
This statement makes a huge assumption, that the DoH provider is more trustworthy than your existing DNS provider. Personally, I trust my ISP (Small, locally owned) with my query history than I trust Google (Massive, exploitative advertising company). The fact that Google is automatically turning this on to scoop up DNS information without users consent should be illegal.
Spock was right, logic clearly dictates that the needs of the many outweigh the needs of the few.