Cut the lifesteal ranged champions get by 66%. Adc's should actively be avoiding any confrontation where they end up needing to take any amount of damage to begin with, so why are we giving them an out where if they live they can continue as though they had never gotten hit in the first place.
If we start cutting the options that marksmen have for self reliance then without any doubt we will stop seeing them in such an absurdly used state.
ADC has the least effect on games now? Why are you bitching
The complaint about marksmen is there's a position in the game called "marksman". They're so vital to any strategy because towers need to die, and marksmen are the best at killing defended towers. There was a brief lapse in the marksman meta when Ziggs bot was popular, and that was the time of the "ADC in 2017" memes.
No, the reason marksmen are so prevalent isn't lifesteal. Mages aren't supposed to be getting hit either, but mages have crowd control and disengage options as a means to protect themselves in addition to 
. It's the fact that they're the only class that can pump out consistent safe damage to both champions and objectives. Blizzard doesn't have this issue in Heroes because forts can be affected by abilities. So your options to fix this would include:
Add more magic penetration items 
to the game. The options for magic penetration are severely limited compared to lethality options 
. This would allow for sustained damage mages to occasionally eclipse marksmen when dealing damage to tanks. Sustained damage mages usually scale better with magic penetration than AP, so such itemization choices would allow champions like these to fulfill the tank killing and Baron shredding roles marksmen fill.
Increase the conversion from Ability Power to auto attack damage dealt to turrets/allow magic damage on-hit effects to affect turrets. This would allow for champions that build Attack Speed with their Ability Power to do every bit as much damage to turrets as a marksman (if not more). Champions that have reason to build 
anyway could sometimes supplant marksmen as the tower killers, so we might see 
replacing marksmen as the primary tower killer.
Rework certain abilities to allow them to hit turrets. We have a couple -- 
Short Fuse and Satchel Charge, and 
Shepherd of Souls and Eulogy of the Isles. Allowing other things to hit turrets, such as 
H-28G Evolution Turret, 
Garden of Thorns, Arise! and 
Void Swarm could allow them to replace marksmen as tower killers as well.
Note that any of these balance changes may require nerfs to the champions involved to prevent them from becoming exceptionally overpowered, specifically champions that already see play anyway such as Azir. However, Azir is a fringe case anyway because he both sees competitive play but is garbage in solo queue. If such changes that buff Azir would go live, I would add a lockout to Q and R while Azir is moving with E. The "Shurima Shuffle" is a highly advanced maneuver that LCS players can perform consistently but non-LCS players cannot exploit effectively, and because of it, should Azir be viable in solo queue (without Shurima Shuffle), he would always be overpowered in LCS (with Shurima Shuffle). Get rid of Shurima Shuffle, and Azir's balance woes can vanish.
Marksmen are supposed to heal right up after a failed assassination. Their weakness is burst, you are supposed to kill them in one rotation.
Which in fact is doable. Assassins, divers, juggernauts, burst mages.... they all got the burst to kill a marksman immediately if they play it right. If a marksman wouldn't be able to heal back up afterwards, this would mean there is no reward for the marksman to survive the assassination.
If the damage just sticks, that means the marksman is so low that he is forced to recall. Meaning as long as you barely touched the marksman, your assassination was successful, since you are forcing the marksman out of the fight.
