Whats the point of prestige points?

Hi, i collected 112 prestige points to get a nice prestige skin. I said that this time it will be riven,...... ANNNND BOOOMM! Again a nice prestige skin for 2000 "prestige" points or better said event points. The only way to get that skin is through paying way to much (200€) , or grinding missions and i highly doubt that on this way you get enough to get a skin. My question is , why are more or less all "good" prestige skins so expensive and the "bad" ones cost "only" 100 prestige points. I think that this is unfair towards all the players who play/main some of the champs that get prestige skins( i am not a riven main ,in this case i talk about riven), are way to expensive and some arent. Will there be a way to get the "event prestige skins" that we missed, or is it just a one time thing. Kind regards , RuneForged