The ability to play on a new account doesn't negate the benefits of permabanning toxic players. If every single one of them did that, then it would at least be a minor punishment, taking away their unlocked skins, icons, champs, and so on. Any who don't do that are removed from the game and the problem is solved in at least those instances. Quite frankly, Riot and 99.994% of the playerbase (people without permabans, according to Riot's figures on this, which I see no reason to doubt) don't give one whit for how permabanned players feel about being ejected from the game for consistently appalling behavior. Most permabanned players stop playing the game. It's extremely rare for a permabanned player to be so oblivious and/or unbalanced that they accumulate multiple permabans on a series of accounts. Riot does not have the wherewithal to prevent that except when a highly-visible player (such as a popular streamer) engages in this behavior, in which case they may issue an ID ban (Riot employees manually ban any account that such a player is seen to use).