This is just one of the little things that makes the game feel more enjoyable, interactive and less toxic. Heroes of the sorm has it, Overwatch has it, so why should League be a step behind?
As of now, i think only Kai'Sa has "Thank you" lines for when she gets healed and shielded, so why shouldn't other champions have this function?
I know the playerbase can be very ungrateful towards their supports, but that does not mean the champions should be.
If Riot could code a special voice line for Aatrox when someone uses mastery emote, i don't think it will be that hard to record and code more "Thank you" voice lines, or just add some to the old champions, provided the voice actors are still avalible.