I think part of the reason side quest has been increasingly ignored is that with the way pokémon works, past level 50~ most mons reach their peak of strength. Unlike the beginning of the game where doing side quest allows one to get stronger mons at low levels, if you have something that works once, it will almost always pull you through on every other major battle, making the casual player not bothering to do the side quest since the reward is not as crucial.
Personal preference. Since side quest are an optional thing, I would like it if side quest was used to give a bit more flavor to the character cast. I know Ame plans to leave those stuff as post-game material, but having character interaction pushed as the last segment of a game seems a tad bit wasteful, as the interactions are suppose to be what draws us to like to characters and care enough to bother to get their content. It'd be nice if we could get to know more about them through optional side quests, players wanting more content will get to know the cast better, casuals will get to progress without having to bother with stuff they don't care about.
Slightly unreasonable I understand, having to plan for this type of story telling would probably murder the development time.