Monster Hunter Generations Ultimate, has 91 large monsters (as opposed to World's 32 and that's including Kulve Taroth which you can't fight right now), has styles which switch up the gameplay pretty dramatically, has G Rank which World is missing, and has the portability of the Switch. There are plenty of reasons why someone would want to play Generations Ultimate after World. Doesn't mean it's better but it objectively has way more content, like WAAAAYYYYY more, so old fans who are used to some of the QoL things from Monster Hunter games of old would definitely have plenty of reasons to switch over to Generations Ultimate especially since World has died down.
I don't even know why this is an argument. I'm sure newer fans are gonna play Generations Ultimate and hate it since it misses a lot of the great QoL changes, but Generations Ultimate and World are very different games and there's plenty of reasons to play one over the other.