Mages only take Aery because it's the closest thing they have to DFG

Literally, you got your champs like Teemo, Singed, Brand, Swain, etc. who used to love that keystone! They're champs who thrived off of damaging over time, and a rune that assisted them in their endeavours was a really nice thing to have! Now that it's gone, all they have to add to their DOT is the Liandry's Torment, while everything else they could utilise is either meant for burst damage or tankiness.

There are a lot of problems with the new rune system that could easily be fixed by having someone at Riot actually looking at it, saying, "Yeah, that was pretty dumb on our part," and making a small, low-effort change to make the game better.

But as we see from that pinned post of theirs, and their responses to everyone's comments in the pinned post of theirs, that's never going to happen.