This is fucking wrong. You think an adc... the guy who's supposed to be CARRYING the damage. This ADC shouldn't be able to kill a tank while they're tanking two nexus towers, 3 minion waves, another player on my team and isn't getting healed? At least 30 aa's to get him under 50% is balanced?
First off, quit changing your story, or else I won't be able to take anything you say as truth. In the OP, you stated:
And my ult + all my combo does 50% of their hp...
But just now you changed it to "30 AAs, both nexus towers, and 3 minion waves, and an ally." Which was it?
Also, yes I believe a tank which you and your team allowed to build pure armor should be able to tank a physical damage carry for an extremely long time. That is literally the whole reason pros pick balanced team comps. You NEVER see a pro team pick all AD outside a few rare cases, and those few rare cases get punished extremely hard by the other team. This is especially true considering you were in the hyper late game. You said you had 28k gold after the match ended, that means any tank should be at full build and then some, even if they were drastically behind you for most of the game. A full build tank is literally designed to tank nexus turrets, minions waves, and champion damage (cuz that's pretty much all they do outside a few cases who have damage that scales off of their durability).
Your second major misconception is that ADCs are supposed to carry their team's damage. This simply isn't true. ADCs are the best tower killers in the game, but they are not necessarily supposed to be the highest damage, they are supposed to be the safest DPS. Any team should have at least 2 damage dealers on it outside some ludicrously over the top "protect the Kog" comp where the supports all stack damage buffs and shields on a hyper carry.
Last thought: maybe you could share the stats/build of this particular tank you were dealing with in this match and it could help us help you better.