so, there's a concurrent post about how 1.1.1.1 had an outage. Does that mean web browsing won't work if that happens again? If not, does that mean it will fallback to unencrypted DNS if 1.1.1.1 is blocked? Making this a default means that Firefox users all bypass censorship in several countries, what do they expect to happen as a result of it? Firefox blocked? 1.1.1.1 blocked?
This isn't any different from if your normal DNS server is blocked, and 1.1.1.1 is not the only encrypted DNS provider. If whatever DNS you use doesn't work, then you cannot resolve any new domain names until you change to another DNS server or it gets fixed. Recently used domains will be cached in will thus still work.
Except for the many domains that set very low TTLs for load balancing and cloudiness reasons.