most played =/= strong.
True, in some cases, not this one tho.
I again, encourage you to do some math. Lvl6 Zed with dirk will take 80% hp of mage with armguard just by R 2Q ele E AA, one more aa and ignite and poof(not to talk about duskblade slow later, so 3Q combo is easier). Lvl2/6 Talon, lvl3/6 Qiyana same things. Yeah, they have to scale... kappa.
Talon is arguably best ad midlaner with how much pressure he can put out just by entering fog of war.
Qiyana is known for her disgusting, undodgeable EQ combo and I expect more nerfs in 9.16.
And Zed might be weakest out of these 3, but he still feasts in soloQ environment, with his sidelane potentiel is something goes wrong.
Until they get a GA or tabi or any armor really.
If adc and assassin are even and adc wins 1v1 it's assassins fault. Rather blame supports.
We'd have much better map coordination if Riot would bring back wards,
Nah, too much vision is forgiving, if I can keep majority of important places on map warded all the time, that isn't healthy.
and then we could revert this weird delayed burst.
What delayed? Their early to mid is strong af. And in terms of kits, only delay is Zeds R and potentionaly Talons R, their combos are done in terms of few secs(1-4).
They're balanced around the idea of snowballing, but that's what everyone does in League now, so they're just melee squishys.
But they are better at that? It's much easier to get kill with assassin than with true scaling champs(assassins don't have to wait to scale, stop it) like Vlad, Ryze, Corki, Azir, Zoe, Cass etc.
Saying that assassins are below S/A tier is straight up stupid.
And how should any other role deal with it? Why is this even a thing in the game? And on a champ who builds attack speed?
Why should they? You said it yourself, "tank > assassins > marksmen > tank" and also stop acting like Vayne is problem, she is putting out only single target dmg, conditional 3 hit proc(2 with guinsoos), with, for adc, low range, her laning phase sucks, she has low cd dash with potential invis and singletarget knockback/conditional stun so she can survive/potentionaly outscale. Not a problem.
, you get a role dedicated to protecting you from assassins and other marksmen, and you don't have to worry about items that rely on enemy levels, like lethality. You just have to get 
or 
on crit-orientated marksmen and you can start chunking people, from 100-0 in 3 shots. And you don't have the option of 1 target, nonono you can just keep auto attacking people and killing them before they can even reach you. What's that? You're an on-hit marksman? Well have no fear! We have something for you too! Just grab this patented 
or 
and they can't even touch you from all the slows! And if you need to kill a tank just grab this 
and 
