Legendary skins for older champions are amazing for the reasons everyone is stating. It's more than just a fresh coat of paint, but the new animations basically breathe life into champions worth PS2 level art and it's amazing.
However, I don't think the issue is new champions getting legendaries as much as it is intense laziness on Riot's part. Look at Odyssey Kayn, it released a year after Kayn who is pretty new and still got completely new animations. Shadow Kayn flips into walls and does way cleaner spins and walking cycles, while Rhaast has new animations to match an uncompromising killing machine.
Riot could've similarly made new animations for Pyke, but nope. He gets emotes only. X&R have new animations on their spells but nowhere near as crazy as some legendaries get. I think Riot has just mandated that less work goes into the skins, like many other areas of the game. It's sad because they let the art departments put out such high quality stuff but I guess it's changing. Splash arts like Qiyana and newer skins (PROJECT especially) just aren't as good and the skins match.
Unique champion designs? Naw, they don't sell skins so no monsters since Rek'sai in 2015 (or 2016 if you count Aurelion). Fun events to hype up players? Naw, let's just throw grindy quests at players. Cool new products to boost revenue? Naw, let's do scummy stuff like take away loading screen borders and certain colored chromas and lock them behind obscene bundle costs so mains feel like they have to pay up to get a complete product, plus rng loot box garbage like Little Legends and event orbs all day long.
Tl;dr it's less of a problem of these skins being for newer champions and more an issue that Riot's standard for art is dropping like other aspects of this game.