“We can replace DNS with blockchain tech... but can we replace IPs?”

If I remember correctly, Nostr does something about it. If you run a relay and your relay has its IP changed, it will try to communicate with the other relays that it knows to share its new IP.

I think relays share their list of known relays to one another, so they would all need to have their IPs changed at the same time for a successful attack on Nostr to work.