DoH in Firefox will bypass your pihole too and have all your DNS data go to cloudflare to boot.

You can disable that; both at a network level, and in the browser itself. You can also choose a different DoH provider if you don't want to use cloudflare.

Yes, it should be disabled by default but it not the end of the world. I would hazard a guess that when Chrome implements something similar that it won't be so easy to disable.

Why couldn't pi-hole answer over http as well? You would also be able to use it from outside your home, without leaking your DNS traffic, or risking interception.

It might work for the better in the long run. No possibility to block ads combined with advertiser's greed will eventually make the whole thing unusable. And I am okay with consuming less content. Maybe along with the fall of the internet advertising industry we'll reinvent the wheel from scratch. Paywalls are fine if you absolutely care about that content but they don't work so well alongside free content back-ed up by ads.