Oh, it can get worse. If Windows market share should really plummet, it won't be replaced by a heterogenous distro utopia but some company like Canonical or Red Hat or a new one will get their distribution to fill the gap. And call me a cynic but I doubt this will be immune to enshittification.
But even that scenario is better than what we have with Microsoft and Apple. The FOSS world would still benefit like it does from the Steam Deck developments.