I agree on the part when you say that assassins should not be able to erase someone from the face of the rift but it's really hard to fix while letting them be assassins, at least when it comes to this game because Riot fucked up most things. I agree that an Assassin should search for disoriented, harmed or badly positioned target but how can that happen in this game? Let's take a look at assassin's primary targets, Marksmens and Mages right? Currently a lot of marksmens are over the edge after kit reworks, qol changes and recent item changes. Adcs are literally overlords with tons of armor penetration and insane amount of crit, other than that, they need a single LS item to life steal up to 30% of their HP out of a single lucky crit. Just pay attention in your games, how often will you see an ADC on HALF hp at the right moment before or after the teamfight? You can't count on "During the teamfight phase" because teamfights are to short and if you just wait to clean up that can end badly for your team. I saw a Caitlyn who chunked down Rengar's hp to 60% with a single headshot and healed up to 30+% of her HP forcing Rengar to retreat..Guy used his long CD ultimate to jump on her just to be hit by a single AA of a ranged carry and retreat even tho they were equally fed at the moment.
I agree assassins should be similar to ''Assassin's creed" assassins but it's impossible at this moment when a God damn Vayne kills you in 3 AA's out of stealth...just imagine how much damage would ADC be able to dish out while assassin is thinking when to attack or waiting for a right moment to initiate. It seems impossible. It would be a good idea if this is a season 3 or 4, but not now in S6.
Riot just gave too much power to assassin's main targets that it is simply impossible to reduce their burst or momentum and keep them viable at the same time.
If they reduce the burst of the assassins they must add some defensive things into their kits and that's something that simply doesn't suit an assassin. They are supposed to be fast, light armored squishy high short burst champions.
Again, I agree with you, completely, but it's not something that would work in league of legends unless Riot completely changes the game, which ain't gonna happen.