Only two assassins have positive winrates (so far)
kha 'zix and shaco sit at exactly 50% winrate . according to leagueofgraphs. the rest are around 47% winrate
Umm. I guess the rework didn't introduce them back into the meta. as the only meta assassin was pretty much just kha zix and some shaco (i think they are also the only ones that had above 50% winrate). and they are the only ones that still do. didn't work out so well. huh. ranged marksman and mages have too much safety in team fights and assassins are still too risky.
I mean there are 5 assassins in the bottom 10 . not much to say
a good way to fix this is to give melee champions higher tenacity.

and
seems undoable.
You also get max cdr with this build and your clone has enough dps to kill squishy enemies in teamfights late game, so you can just Q in, R and order the clone to attack the adc/mid etc and the enemy team will focus it, when the clone explodes you can finish your target.