I can certainly see a world where the market doesn't fix this problem.

Same. Though we also have to distinguish between long-run and short-run. If the duopoly becomes too shitty, they'll still maintain dominance in the short-run, but in the long-run a better alternative is likely to emerge.

The level of their enshittification also depends on the viability of alternatives. If viable alternatives exist, even if they're niche, it could prevent the duopoly from becoming too crappy.