I think it's a question of adoption.

If bitcoin stagnates and stays at its current level of users, then in 15-20 years, I would be concerned about its ability to remain censorship resistant. The cost to censor or otherwise disrupt the network would likely be quite low.

However, if bulls are right and bitcoin has 5x, 10x, 20x or more people using it in 15-20 years, then I don't see any problems with a fee-only model. There isn't enough blockspace for everyone to be on-chain, and so if the network is in demand, there will be a cost to use it.