But isn't it a problem that a 25 episode Anime still hasn't gotten to the core of its story (the royal selection and the reason why Subaru is involved)? It's like we watched 25 episodes of introduction to a story we're maybe never even getting.

That's what you think the core of story of is. Besides everyting is part of story because their all related to Satella both directly and indirectly. A good large scale story span would be buliding these core aspects of the through a multiude subplots reavel the mysteries of series bit by bit and not throwing at because the author wants to get over and done. Even mainstreans lik LOTR and HP a lot of central ate handled in small bits and bulit up overtime. How long something is not explain is really not as long actually addressed at one point. Beyond that it us just an arbitrary metric.

Fact of the matter is Re:Zero wasn't written to be finished in 25 eps and that has nothing to do with as mich does the TV format. You're gonna have to deal with that as it is the curse of most adaptations.