Prestige Skins aren't "new", they're just the next escalation as Riot tries to make skin sales still a good thing.
Now, I don't mean disrespect in terms of how skins look. Skins today blow skins of previous years out of the water. That's the exact problem though: There's a nuclear arms race for skins. Let me ask you, reader: How many skins do you have for your favorite champions? Barring a ban or an account loss, you're probably rocking 1-3 skins for most of your favorite champs right? Or, on average, you have the skins you really want and nothing else.
This exact problem leads to Riot always having to go bigger and bigger. Skins have to be rarer and rarer like Prestige to justify players buying them and still feeling accomplished. Skins now have crazy non-chroma variants (The duo Blitz skins, Solar/Lunar Eclipse Leona) to get you to buy similar skins twice. Skin themes/universes are propped up with a similar amount of care and love to the main universe. There is an arms race going on as we speak of Riot constantly putting more and more effort to get you to buy a skin for a champ you likely already own because it's the next, cool thing you didn't know you wanted.
Riot, if you want to make money, there are tons of avenues you have yet to explore that wouldn't require half the work. We still don't have a lot of things other games have: Announcers. Finishers. Map variants/accents. Minion/Tower Skins. User Interface skins. Hell, DOTA once sold weather effects. The common counterargument is "ugh then my game will be a clarity nightmare", but that is countered by having it disabled in ranked or only affect the player on player's side.
This constant escalation of skins isn't healthy nor is it sustainable. Prestige is just the newest tool in a long line of skins to try and get players who already have every skin they want enticed with promises of "New free stuff". You have so many other avenues to explore and so much more you can do that focusing on skins isn't just a bad decision, it's borderline unhealthy for the rest of the game.
TL;DR Stop focusing on skins, give us some stuff that's not skins. People more willing to spend money on something they don't have/something new than to justify buying a skin for a champ who they already have 39 skins for.