For lots of people time is money, so once major attractions are longer than a 30 minute wait, I'd argue that a paid system is viable. I'd also argue that one reason why wait times are so low is because there's no fast pass. Allowing people to cut the standard que has a negative impact on the standard que and once they add something back in, wait times will be going up. There's lots of ways that they could implement it and they're still using the fast pass system for things like guest recovery, disability services and (I think) rider swap. I mean, they could use it as an incentive to stay at a Walt Disney World Hotel. Say 1 anytime fast pass per day for staying at a value, 2 for moderates and 3 for deluxe. They already did something similar for people staying concierge.
Under that criteria, I'd say that it would currently be viable at Magic Kingdom and Disney's Hollywood Studios. They don't even necessarily need a high tech solution right now, they could just as easily give people lanyards with the date on it. The could easily just say that they system is evolving right now, so people know it's not necessarily going to be something that they do long term. Just the advantage of not being in a que with a lot of other people may be enough to get people to pay for front of the line access, it was for me when I went to Dollywood this July when the longest wait was 30-45 minutes. The advantage to a low tech, front of the line system is that it's almost all pure profit.
It will be interesting to see what they do, but I don't think it's going to be exactly the same as it was before. COVID has cost them way too much money for them not to try to find new ways to make money.