Lifesteal heals more the more damage you deal so it's actually only really valuable against enemies with little armor. Due to this lifesteal is extremely valuable vs minions/monsters since most of them have 0 armor.
Now about 25-75% of lifesteal on minions is due to overkilling them. If attacks wouldn't be able to overkill any units (if they have 10 HP you will deal 10 damage) then lifesteal loses a lot of its value especially against minions but also squishy champions.
No overkill -> less healing
Nerfs to stats are all about making them lose value. This change would make it lose value only against what it's currently good at.
Also it's called lifesteal. You can't steal life that's not there.
External link →