I dispute that. Enshitification happens when the user does not pay for the product. The provider of the service has to find alternative ways of monetization. As a user, this feels shitty, but you get what you pay for. With AI, if you pay for usage, you can demand that the service provided not be shitty. You can even litigate, but the most important leverage is the threat to take your business elsewhere. Enough people do that and the provided service stops being shitty in a hurry.
How many providers do you have to pay for to watch your favorite teams play in all their competitions now vs 20 years ago?
How different is the experience booking an economy flight today vs 20 years ago?
I think you are unduly optimistic. There is little that prevents it from happening to a product that you pay for. Airline flights, for example.
All it takes is cost-conscious customers who will accept a worse product for less money. There will be competition to see how who can go farther down the "worse" curve.