They used to do this in FFXIV PvP too, hell it was copied from PvE where it's still the norm (diminishing returns on all status ailments like paralyze or sleep). It would make some sense to apply it to FL at least, given the crowd control for PvP is balanced via 5v5 matches. Maybe they decided against it because purify is available to everyone, but it was in the original version of the mode too, players just had to spec for it.

I would personally find this solution much less soulless and boring than just kneecapping AoE or DRK specifically by culling max targets to 5; I hated that choice in PvE since HW (and AoE is far more trivializing for dungeons nowadays than it was then anyways), and I hated it in WoW, too. Hitting big groups, doing big damage and making big plays is one of the strengths of the MMO medium. Don't just toss that out without trying anything else.