considering most people today only use 3 or 4 big email providers i can see the exact same happening for nostr, the p2p part seems more like a gimmick than a protocol requirement

It's actually advised to use 3/4 relays in your clients

Using NIP-65 (https://github.com/nostr-protocol/nips/blob/master/65.md) you can broadcast a note to the network to mention your preferred relays.

Most clients fetch that note when you first sign in and make sure you connect to your preferred relays