What do you mean unsustainable? It is sustainable as long as the administrator is willing to pay for the cost of the servers out if their pockets. Lots of people run websites, forums for years all paid by their own pockets. The free relays have been running servers for free out of their own pocket, they just purged the free relay when the hard drive becomes free. I would never pay for a service such as a relay out of my own pocket. It just attracts spammers, scammers and freeloaders. The free relays operators have said they want to keep the relays free or it will push away new users because new users are not willing to pay for anything and it will slow down the growth of nostr. This is debatable what what we can do is education users once they are comfortable using nostr to move over to the paid relays. Alot of this issue would be solved if free relays just added a small fee of 21 sats or lower to simply use the relay. In the long run once Nostr is much more popular free relays will disappear and I believe a consensus can be reached by writing in the software that only relays that adopt a small registration fee can be part of the network. A free internet is no longer the future going forward. It will still exist but the free internet will be just spam.