Why are we nerfing Mercurial Scimitar?

I just want some context. I feel like Mercurial Scimitar already feels like crap - you HAVE to build it in retaliation to certain champions on the enemy team. If those champions weren't there, you wouldn't build it. Other items are simply too much better that if you're not in a situation that explicitly calls for QSS (so you're not against Zed, Malz, Liss, WW, Skarner, debatably Nasus, etc...) then there's no point to build that item over Bloodthirster or even the soon-to-be-weakened Maw of Malmortius.

So why are we nerfing it further? It's not like all (or ANY) of the above listed champions are struggling right now - Malzahar is the most winning-ness mid laner, Zed's a terror that just got nerfed, Lissandra is somewhat weaker but is still annoying, Nasus, WW and Skarner and all doing extremely well. So we're indirectly buffing a bunch of champions that probably are on the verge of needing nerfs. And for what reason? I haven't seen anyone complain that Mercurial is overpowered or bought too universally. It's a counter to specific champions and is only really any good in those situations.

If there's a reason for it - great! I can't think of anything though. I get that it's only 5AD being taken away, but it seems completely unnecessary.