An assassin getting a couple of early kills should not turn the game into an unwinnable clusterfuck

If an assassin gets slightly ahead, you completely lose the ability to interact with them, because any interaction will result in you dying. If you're playing a caster, being even with the assassin also isn't enough; they'll one shot you with less gold. You have to be severely ahead in order to match with them, and even then, they still have a major advantage in roaming, because you can't really follow them for a variety of reasons.

Mid lane has been dominated by assassins for the entire season because of this issue, and with the way things are looking, we're going to have another season of this. Currently, of the 15 most picked mid laners at Diamond, 10 are assassins and make up 65% of all mid picks, while 4/5 of the remaining champions are below 50% win rate.

This is especially problematic in scenarios where the teams is even on kills, but all of one team's gold is consolidated on an assassin. The assassin can use this gold much more effectively than anyone else can, because the more gold directly translates into less counterplay for their opponents.

One of the big issues is that 95% of mid laners cannot build a defensive first item, period. This is not remotely viable at high elo; if you look up statistics for casters rushing a defensive item like Veil or Hourglass, they almost always have 35-40% win rate with it. It's more efficient to continue to build damage and hope you can capitalize on a mistake and kill them before they kill you.

It's also a major problem for jungles. Unless you're playing a tank, that slightly fed assassin can catch you in your jungle and one shot you. It's also incredibly frustrating to watch them roam and 4-man gank with the jungle, but being completely helpless to stop it, because you know that you can't duel them and your mid laner can't follow.

This is something that really needs to be improved for the pre-season, whether it be reducing their early game impact, or improving defensive items for casters, so that rushing defense is more viable than it is now. 3 kills should not be enough of an advantage to set the pace of the game.