If Wikipedia becomes unfashionable to the extent it is highly dependent on its savings, it will no be longer valued for being Wikipedia (by definition).

It might be valued as an institution with modest economic power, which doesn't seem to me to add anything to the world that isn't already there.

2 years of all current spending as a cushion seems like plenty to me (for Wikipedia, which has not faced any unexpected financial shocks, even in 2020). If you can't navigate shocks and downward trends with that cushion, then you probably ought not to exist.