Do you know why you are getting nuked hard in an unfavorable meta? Because in the opposite situation you are untouchable. The ADC class has the best damage:counter play ratio out of any class and is therefore immensely valuable. "Late game insurance" has nothing to do with it; marksmen were still must pick back in season 2 when some fighters scaled equally as hard as a marksman. The only counter play to most marksmen is to remove them from the game. In a favorable meta this isn't even an option; tank meta means you stack tanks, one control mage and maybe an Enchanter and you let the two ADCs stat check down the rest of the team. Enchanter meta - same result.
And before someone says "well reduce the damage and make them tankier." One, almost none of you actually talk about this until you're confronted about asking for buffs to the mandatory class. Two, this will not change anything and honestly will probably make the game worse. You still have no mechanical counter play beyond getting removed from the equation entirely. Tankier ADC is just an attempt to sideline the issue; unless you are completely abandoned by your team most enemies can't do anything to you once you become relevant. Because this game is now DESIGNED around you being strong damage but extremely vulnerable. It's what assassins are designed to exploit. It's what enchanters are designed to negate. You have two classes minimum that exist solely around the premise that you are easy to kill. You cant change that on a whim.
Marksman have never and will probably never be completely irrelevant unless critical strike systems change massively or the champions get reworked to the same standards that carries from other classes have been getting. You know what happens when fighters or assassins are bad? They don't get picked. At ALL. Marksmen have never had this issue; one kind of marksman ends up weak so you pick another. Late game not happening? We pick Lucian and Draven. Early game doesn't matter? Bust out the Kog Maw. Utility carry meta? Sivir and Ashe. This is not a problem of marksmen being pushed out by assassins or mages; marksmen are just as guilty of doing this to each other as anyone else and we have 9 years of history to prove it.
Marksmen are simultaneously a compulsive pick and yet they are completely at the mercy of the game around them. Marksmen are almost never viable on their own merits and always become strong or weak as a response to the rest of the game, including which ADC itemization is strongest. Because they HAVE to be with the way they work. Yeah, the game doesn't always suck you off. It happens to all of us. You'll get your turn again; we all do. Fucking around with crit for the third time does not change this in the long term; if you want this dynamic to change then the champions need to change directly.
This isn't to put people down for not enjoying which champions are good right now. It's perfectly fine to want your champion to be better. But if you want a systemic change then you need to address the real issues surrounding the class. Issues that throwing numbers around on a slider will not solve.