Unfortunately this is an issue of having budget limitations and yearly cadence. Even V reuses lots of assets from Z3. It's the curse of this series: those HD sprites and animations take lots of effort to make and the team wants to use them as long as possible
I think my main issue with Y's asset reuse (which, let's be serious, on the player side there is still a lot but at least most units look very good) is with the voice acting: yeah retired and dead VAs are an issue, but lots of playable characters have VAs that are perfectly capable of working and yet were not called to record a single new line. Those things make you realize the game simply had its budget slashed, and still I'm conflicted because considering how many titles Bamco straight up cancelled last year it's a miracle that Y came out, and it's so good despite the obvious cuts