Ash's Rowlet doesn't necessarily EAT the Everstone, he kind of just swallows it. I would guess that Rowlet doesn't not have enough time to evolve while using Seed Bomb, so that's why he never evolves. I think all his other Pokémon that didn't evolve (Squirtle, Bulbasaur, Snivy, and Oshawott. Probably others I don't recall too.) just didn't naturally. With the exception of Bulbasaur, since it was starting to evolve and then forced itself to stop (there are theories to why Bulbasaur and Squirtle didn't evolve, like that they were scared to evolve because they saw what happened when Charmeleon did.). I think that Ash's Rowlet would have evolved if it didn't have an Everstone, maybe while battling Hau in the Alola League. I guess we can't be certain since Rowlet didn't evolve before having the Everstone, meaning he will never be a Dartrix or Decidueye.
So he will never evolve because of his Everstone and no one will ever know if he could have. Hope this helps!!!