Your reply was:
> But to say there is no reason for an ISP to serve DNS is absurd.
The part of my comment that you quoted does not at all say that. (In fact, no part of my comment says, or even suggest, that.)
I agree that any DNS provider can abuse your data, but it's important to look at incentives. Comcast doesn't care one bit about its public image because it already has a terrible one, but customers have no choice in the matter, so Comcast's public image is mostly irrelevant.
Several DoH providers bill themselves as privacy-focused, and make privacy a big point in their marketing around their DNS service. Violating that privacy would be damaging to their product and reputation, in a way that they'd likely care quite a bit about.
I'd rather just have my local resolver have a list of 5 or so DNS providers with reasonably low-latency presence in my area (possibly including the local ISP, even, who knows), and just round-robin requests to them. There's really no way to make DNS not an "open book" as you put it; you can't ask someone to resolve a hostname for you without telling them what the hostname is.
So yes, we need ways to mitigate harm. Unless a provider has their reputation on the line, I don't really see a way to keep providers from doing sketchy things with your data, at least not without legal regulation. It's not like things like the EU's GDPR and California's CCPA were dreamed out of nothing; they came about because people have started to realize that companies just will not act as good stewards of our data unless we legally mandate toothy financial consequences as punishment.