There are many countries where ISPs are obliged by law to spy on users, and retain logs for many years. DNS manipulation also used as a cheap censorship mechanism. So Cloudflare easily can be a better option for hundreds of millions if not billions of people. As a rule, local actors present way more serious threat compared to US agencies for majority of the planet's population. That said, Mozilla, of course, must be very transparent with such big changes, and explain them to users not using just "more secure" wording.