I know that we are in dmg meta, but I want to share my thoughts with you.
Assassin role is a champion who should be highrisk highreward playstyle. As a role definition, he should be able to kill enemy squishy-carry and DIE, or escape and wait for cooldowns.
Now situation is pretty different.
Assassins are more like bruiserassassins, that are still dealing tons of dmg, with huge mobility and way too big survi. Also I have to mention, that assassins are one of most snowballing champions ingame. So... it is very easy to get early kills for most of the assassin-champions plus his snowball potential is equal to carry a game.
Also we should mention, that most of assassin champions are just uncountrable or barelycountrable.
Few metas ago assassins were falling off in late game, but in this moment when games last up to 40 minuts, not up to 80, this option has no place over there.
Assassins are just too strong, with too easy escape, too easy dmg potential and way too easy lanephase.