The thing that defines it to be a Nash equilibrium: namely, a Nash equilibrium is such, that for any individual entity, deviating from the strategy that consists of the Nash equilibrium is less advantageous than keeping with it. So once a system is in a Nash equilibrium, no individual entity has any incentive to leave it if it acts alone without collusion.
A Nash Equilibruim suggests there are different strategy profiles. In the context you present there are only ONE set of players, the miners.
Yes, you are right, the system is more complex, and there are also the users (people buying and selling coins essentially). But the Nash equilibrium is essentially this one, where the miners can only stick to the original protocol
Why? Who evaluates that they have done so?
The miners amongst themselves.
Of course, this equilibrium can be broken, which is exactly the case of "forking away" and hoping for success in the market, but it needs a form of collusion between certain miners, and enough users.
As I said many times, this is a NATURAL way of evolving crypto: forking away, taking the risk in the market. With a PoW coin, this has to be initiated by miners.
Yeah? Watch UASF
Obviously, a UASF node comes to a grinding halt UNLESS miners decided to fork off. The whole UASF is nothing else but a kind of shouting contest to put some psychological pressure on some miners to go ahead and make them decide to fork off. Without miners deciding to fork, UASF has not much to it. The "user" part in UASF is similar to the "democratic" part in the old Eastern-Germany name: "Deutsche Democratische Republik".
Users only come into play with a UASF after:
1) miners decided to fork off
2) exchanges decided to list both coins.
Only at that point, the users can vote with their money on the exchanges to pump up one of the coins, and to dump the other one (taking the risk that they are betting on the wrong horse in doing so).
Economic actors do technical analysis on coinmarketcap, or use any other betting strategy. The debasement is never part of these considerations apart psychologically maybe. The market price includes already all debasement
How does the market come to learn about debasement?
Increasing offer should decrease the price.