A few central relays, yeah that's extremely likely. Nostr.wine is already heading in that direction. That's not in and of itself a bad thing. Especially since the central relays will be surrounded by a sea of competition ready and waiting to take over should the central players start playing dirty.
Similar to how there only a few mining pools with most of the hash power, but the instant any of them act up, they lose all of it as the actual miners send their hash rate elsewhere.
If everyone is using just 3 relays it’s not that trivial to switch relays.
Why would you connect to a new relay if nobody is posting there?
Why post there if nobody is listening to it?
If it's just 3 relays, then yeah we're in trouble. If it's about 20 relays with most of the traffic, and a few hundred around them, then it's probably alright.
There's also no real reason users couldn't hold maybe a few hundred megs of their own event history, post that to relays in case. Plenty of ways to deal with this kinda thing.
