Spellvamp was removed because it didn't really work for how casters work (with noteable exceptions like Vlad and Mordekaiser). Most casters want to go in, burst, and get out while their skills are on cooldown. Spell vamp doesn't do anything for you there. You either got caught and died, or you got your stuff off and got out. It created this weird clash where it doesn't really work unless you stagger your spells which wasn't optimal. Removing it allowed them to balance those that relied on it (Vlad), and didn't really hurt those who didn't need it.
Lifesteal on the other hand goes really well with the patterns people would already do. ADCs want to be constantly autoing, which works well with lifesteal and is synergistic, not clashing like spellvamp did. Lifesteal already is hard to itemize more than one, maybe two items. To get the 45% you state, you have to have Mercurial, BoTRK AND Bloodthirster. That means if you have boots you only have 2 item slots left. Now you have two item slots to get Armor pen and crit.
I'm not against capping lifesteal if they found the right balance, but its not as ridiculous as you make it seem.