They're just offering a lot of content recently. I don't think that's a bad thing. If something is for sale, that's always better than it not being for sale.
The prestige skins are a bit more than chromas, they have more visual effects and altered audio effects. At least that was what I got from Prestige Kai'Sa.
So they are subtly (subtley?) different to chromas. I understand if you feel like your welfare as a consumer is going down because it would just be a 290 rp chroma if it was released last year (as all the bells and whistles would be on the base skin in the first place and the colours would be the only change). So you miss out if you can't afford it, or feel ripped off if you can afford it. You also probably hate this direction in general.
I feel like this is just Riot recognising a segment of players that collect skins and are offering this content to them. Riot probably make most of their revenue in RP from these players and as a profit maximising firm they will always cater to this segment of players. I don't see that changing.