Why is there competitive gen 5, it shouldn't exist with all the UB mons?

Ggbri66

I mean with amoongus existed its wondered if gen 5 is an obsolete roster, i mean ferrothorn was short lived but amoongus that lived until counters

Dougbro1

There are niches that some play that can't simply be filled in by new 'mons. Amoonguss is one example, Landorus Therian is another. I don't see what roles the Ultra Beasts play that could replace these.