Should lifesteal be changed to health regen?

I thought the purpose of any sustain would be for classes that sit in the thick of combat and not on the outskirts, but in LoL it's the opposite way around?

If Riot wanted ADC's to have their HP ready at the start of every next fight and not drain tank during, why not change their itemisation to regen so their healing doesn't scale out of control with their damage and is easier to tune separately?

Then put lifesteal on defensive items, so champs that are in the thick of it can sustain better without having to inflate defensive stats to keep up with how ridiculous damage has gotten, the lifesteal won't be oppressive because they're not building damage, fulfilling its purpose of giving a little and not a lot of sustain while in combat.