I think the problem is, where to stop. They also said they don't plan to unretire the Sprites. They also said they won't use user suggested breeds (in the Q&A recently). So should they delete all those threads as well?

I think beside giving concrete suggestions, those threads also show some general concerns of some of the users.
Like:
- threads about bringing back KS and old festival stuff keep reappearing -> Maybe there shouldn't exist too many retired items. Let's try not retiring too many items by introducing Joxar's box.
- suggested water breeds with fins keep reappearing -> maybe the next breed should be a water breed

Also if you delete the really impossible ones, the other people might get their hopes up for their suggestions that aren't deleted even though those won't make it either just because the staff decides that they don't fit FR.

So it's better to not delete anything I think.