Except Renekton is specifically tuned to thrive in crowds. Not everyone is balanced around that, and some of the champions who are (
,
) are also responsible for spell vamp being less powerful than it could be. I also don't think the idea of reducing effectiveness on multiple targets, whether they're champions or minions, is a good way of solving the problem of AoE healing, because you're inevitably still getting more healing (and potentially a lot more) than if your ability targeted a single opponent.
While I agree that spell vamp doesn't have that much clarity, I disagree that the same applies to life steal. The fact that it applies only on basic attacks, or abilities that mimic basic attacks, means there's a clear way of knowing where a champion's healing is coming from, and at what pace.
I don't think the answer to the AD/AP sustain difference is to necessarily homogenize the two. For example, AP champions tend to roam a lot more during the mid game, so it might make more sense for them to have stronger regeneration than AD champions, which is already the case. I also don't think it would be wise to have AP champions sustain off of minions in the same way as AD champions, since AD champions cannot clear a wave instantly and heal off of every minion at the same time, whereas an AP champion can using spell vamp.
While I agree that even AoE champions struggle to make use of spellvamp, I think that just means spell-based sustain for AP champions needs to be gated independently of how many targets you hit. With your model, any amount of sustain you have planned for single-target champions will inevitably multiply itself for AoE champions, so you can't really have your cake and eat it too.