First off, several items, some of which you've listed, have Lifesteal as a UNIQUE Passive. It's purpose is so they can't stack the same item.
Lifesteal is a UNIQUE Passive, building 2 or more won't increase the amount of health gained.
Lifesteal is a UNIQUE Passive, building 2 or more won't increase the amount of Lifesteal gained.
Lifesteal is a stat.
Lifesteal is a stat.
I'm guessing you mean a "UNIQUE Passive - Named", such as Sheen and its upgrades with Spellblade, or Last Whisper and its upgrades.
Buying Lich Bane, Trinity Force, and Iceborn Gauntlet will only proc "UNIQUE Passive - Spellblade" from the one with priority (in this case, Lich Bane).
Buying Last Whisper, Lord Dominik's Regards, and Mortal Reminder will only proc "UNIQUE Passive - Last Whisper" once. Could you imagine someone with +135% Bonus Armor Penetration?
Now, let's say every item with some form of lifesteal effect becomes... "UNIQUE Passive - Lifesteal". What changes?
Not a whole lot. Most champions (with a few exceptions) tend to only purchase one Lifesteal item. It's a poor stat on its own as it needs damage to really be effective.
15% Lifesteal out of 100 damage is only going to heal 15 health. 15% Lifesteal out of 1000 damage will be healing 150 health.
My verdict? Turning all Lifesteal into "UNIQUE Passive - Lifesteal" wouldn't change much of anything.