It's a tradeoff and people should weigh the pros/cons seriously. Firefox claims it shows a notification popup, but it may be too easy to click away. (I didn't see it.)
It should be front-and-center.
Big claim, with no evidence what so ever. Consider this a "Citation needed".
https://en.wikipedia.org/wiki/DNS_hijacking#Manipulation_by_...
Unless they explicitly say so, your ISP is not spending a boat-load of money running a DNS server for you to use because they're nice. They're not even doing it because you're paying them for a service. They're doing it because they can monetize the data and serve you ads.
Nope. You have to provide the address of a resolver, but that doesn't mean you have to run one yourself. There's nothing stopping the ISPs shipping a standard DHCP config that points their customers at (for example) 8.8.8.8, or 1.1.1.1, or whoever.
A few minutes walk from me some people opened a gelato place. You go there, you buy some gelato. Nice. Warm summer evening, drop in, buy a cone, delicious. I am 100% certain that by selling me frozen desert they get money! Their plan may not specifically have focused on me enjoying this at all. And yet, since the effect is that I can enjoy desert that's exactly what I do, and I don't begrudge them their money.
For many, maybe even most, the situation is reversed. ISP-provided DNS (the default for 99% of web users) is very often intentionally mis-configured to return ad-laden "search results" instead of NXDOMAIN. The situation is more authoritarian administrative districts is even worse. You're right that trusting Cloudflare isn't ideal either, but they are at least better behaved than most ISPs, so it's the lesser of two evils, I think. Non-technical users shouldn't be expected to know how these things function, they should just get the least-bad option by default.