Mages one shorting is not too surprising and seems extremly common. Let me tell you why lux isn't broken as a 800k lux main.
1- She got no escapes
2- She's completely skillshot oriented and her missile speed is decent
3- One single miss with her Q can result to death
4- After one full combo, she's useless for a few seconds waiting for her abilities to go back up
5- No escape means no gap closer meaning you can escape her easier if you're not caught in her Q compare to mid laners that does have gap closer such as zed, talon, le blanc, etc
6- If you miss your ult in a teamfight, or waste it on a zhonya'd/qss'd target, your team could just straight out lose that fight. QSS and cleanse users are a huge pain in the butt!
7- I single cleanse or QSS can straight up avoid her full combo and when they can use that small window to get close to you while your abilities are down, you could be dead.
I'm sure I'm missing other cons but because of these cons, nerfing her damage is gutting her too much. Her cons make her burst damage acceptable.