Instead of asking why MF's Grevious Wounds was removed...

We should be asking why it's still in the game at all. It's an incredibly binary system that both feels bad for everyone involved and is bad for the game.

If healing and sustain are balanced around the idea that GW is applied: If GW is applied, it is working as intended and everything is fine. If GW is not applied, then they are healing for far more than can be done to them, and your team can only beg your AP/support to buy Morello's.

If healing and sustain is balanced around GW not being applied: If GW is not applied, it is working as intended and everything is fine. If GW is applied, than you have a bunch of really sad Vladmir/Mundo/Soraka players who just got countered by a 2300 gold item.

It's probably time to straight up remove GW. It doesn't provide any meaningful gameplay for most champions (except maybe for Mundo?) and is a product of a bygone era, where healing was more common and GW was on more things.

(I also feel the same way about armor/mr pen, especially % armor/mr pen, but that's so ingrained in the game that it's gonna be impossible to remove barring massive changes. GW is already almost out the door, anyway.)