I'd much rather just have long/permanent chat restrictions than permabans
Permanent bans help nothing. The toxic player will just create a new account and continue to spread the poison. That or you might end up with the occasional psycho who'd fly down to Riot HQ with an M4 Carbine. (Hey, it happens with post offices, why can't it happen to Riot?)
Why aren't you permitting people who want it to just have super-long or even permanent chat restrictions?