The AS buff that Lux got doesn't make her passive easier to use against Champions
Even if it might help a bit with farming, it does not change the fact that once the enemy hits level 6, Lux players will NEVER try to autoattack them. If she tries to proc Illumination on a post-6 opponent with an auto attack, she's asking them to jump on her and kill her.
Is it intended that Lux simply not use her passive in mid-to-late game against Champions outside of her ultimate? If so, that feels like terrible design planning.
vs
1v1, the risk is totally worth taking if Amumu has already used his Q.