The lifesteal issue with ADCs early in lane isnt that big of a problem in my opinion (it does become an issue mid/late game I believe). This is because both ADCs are doing it. On top of them both life stealing, sometimes they even have a support that offers sustain, but the other one can have it too.
For Morgs case, not both mid laners have some kind of excessive sustain mechanic. When I take Morgana mid, I relye upon sustain to help keep my opponet disinterested in attacking me. The changes just make that sustain something my oponent can see coming and try to work around.
In Nasus case, he is able to be shut down hard by a handful of champions, and his sustain puts him in a position to be strong against other champions. So there is an excuse for letting him lifesteal off of minions. Hes immobile, has one CC ability, and has to farm (limited by the cooldown of an ability) to have a mid game threat. If he couldnt keep his HP topped off he wouldnt have much of a laning phase.