Both those champions have TONS of waveclear.
Lux deletes waves with her E
Veigar Deletes waves with W
And as a person who plays a TON of Karthus, Karthus E straight up makes him a decent splitpusher.
Every Mage, without exception, has potent waveclear.
All of them....well, except Morganna since she has been tuned to being a support. Go figure.
and as for Morg Shield strentgh, that isn't even alot!
Not only is she maxing it last in midlane, but even if she was. 3500 is a joke.
Lux E at max rank is 60 / 105 / 150 / 195 / 240 (+ 60% AP)
meaning all it takes you is to have even just
and you pop the thing in 1 shot. And Morg isn't goign windspeakers unless she is playing support, in which case, she is not only maxing Q, but is not building enough AP to make the shield matter past your damage.
This isn't about "whether you beat Ahri in lane or not. I beat Lucian in lane but that doesn't mean he isn't currently overtuned.