How are marksmen not abusing lifesteal exactly the same as vlad and swain "abused" spell vamp?

So what exactly changed? Because marksmen didn't really get significantly more lifesteal in that time, but even before the new rune they were getting insane healing from lifesteal, enough to heal them enough to put them on part with juggernaut durability in many cases, let alone talking about the infinite sustain that it offers.

It's honestly just how much damage is being done now. The amount of damage that marksmen have to output to be competitive is so disgustingly high, that having a direct damage to healing conversion on their primary source of damage makes them nearly impossible to punish for mistakes, unless you're playing an equally excessively high burst assassin... or

If it was justified to remove spell vamp, because it was impossible to balance given champions like vlad or swain, despite a 33% reduction to any form of AoE damage (which is virtually every spell), then there's no way that marksmen can't be said to be abusing lifesteal in the same way that vlad or swain abused spell vamp.

Which, side note, no longer works as an excuse. Both have been reworked, and if riot ever wanted to give all the other AP champions access to spell vamp again, without it being abused by those two, this was the time to fix it and reintroduce spell vamp. But they still haven't.