But if the iOS store is so much worse for devs than the Play store, wouldn’t there be more incentive for devs to push for a legitimate alt store to succeed on iOS?

Android's third-party app stores have been majorly gimped for a long time unless they were installed by the manufacturer. Further, they don't even have great reach even if they're installed as such, as evidenced by Epic wanting Fortnite back on the Play Store despite it being available on the Galaxy store.