Not every single, just the big ones - which most users subscribe too. Probaly less than 20. It's not impossible.
Ideally ppl would publish to their own respective relays, and clients would find them via DHT-style discovery. But that doesnt scale I guess. Hence the current middle ground (some big relays as central access point for 95% of notes)
Even if that's the case (#amethyst for example can connect to 160 relays and still work) there are too many ways to overpass that "censorship". Even ways that are still not defined yet.
The kind of problems you only worry about while no one has it.
You are also missing the Streisand effect. If it's proven than any of those 20 relays are running an internal black list. Everybody will just ran out of them.
