Just my experience playing mage mids vs "OP Assassins"
Picked Anivia, against Zed Level 2, q him while he aas minion, into e aa. Half his hp. He chugs a potion, I do it again, drop ignite and he dies. Next, he gets 6. Ults me, landing 2 aas, 2 qs, ignite, and e. Very barely pops my passive, I only die due to xin coming in. Level 9, he ults. He doesn't land q, uses ignite. Believe it or not, he didn't actually deal 200% of my hp.
Yes, if he manages to weq you, he will do 50% of your hp. So will literally every viable mid in the game, typically a lot easier than he can.
(Edit, I'm adding a bunch of additional examples, as one game probably isn't the best sample size.) Cass vs Irelia, she never managed to solo kill me. Every time she would engage, I would dodge E, and she would either back off or dash in recklessly, I would q and kite, dealing a ton of damage with E. Considering Cass is counted as "Late Game" past levels 1/2, it wasn't exactly like I had a huge advantage on her. I even managed to solokill her at 6 with my ult. We did lose that game, but it had nothing to do with the mid laner. Ahri vs Akali, she never managed to kill me. Literally just by not standing in the shroud, it was really difficult to die. Plus, one lucky charm and she was essentially dead. Very very few assassins have abilities which are land and they die, of course they do have their target access, but a good wall flash will save you. There are plenty of games where the opposite happened; Cass vs Ekko, I was doing perfectly fine until he got protobelt. From that point on, he could 100-20 me consistently, forcing me to farm under turret and play super passive until I got jungle ganks. It does all depend on player skill, champions, etc, but it isn't like assassins are so broken that it's not viable to play mages.
just an opinion and whatnot