7sec to kill someone that did buy nothing but defense on you're own is rather short, and once you get teamfights, the damage gets stacked up and Tanks end up dieing in the duration of a single hard CC.
Unless i'm fed on a tank or bruiser, i never feel tanky no matter how much i invest into defense.
The issue is not just marksmen shredding tanks, as you said, it's what they are supposed to do, the issue is that everything does solid damage to tanks and adc atleast once they hit lategame are a bit to good at shreding tanks.
Compared to S5 or even worse to S2, that's a massive difference. Not only would Tanks survive a bit longer vs ADC, there was also less damage from the rest. Especially in S2, tanks and supports could basically be ignored while mages and assasins had more CDs so less dps and they'd try to not waste the CDs on the tank. That allowd tanks to initiate fights vs 5 and actually survive without **imidiate ** followe up.
But buy now, all tanks have %hp damage, mages have noticable dps, overall downtimes are rather low and supports have more gold which often is spend on damage, either directly or indirectly through stuff like ardent or zekes while defensive items have been slowly nerfed over the years, especially in terms of sustain while anti tank got boosted(liandries buffs, BotrK is stronger than ever, CONQUEROR, IE granting truedamage..)
The issue** isn't that assasins or burst mages can burst full glasscannon squishys, that happend in S2 aswell and is totally fine.
The issue isn't **that bruisers that invested some gold into damage can somewhat quickly kill a squishy with no gold spend on defense.
The issue is that supposdly tanky champs don't feel like they life any longer than squishys unless the snowball
The issue is that tanks can take down squishys in nearly one rotation with no gold spend on damage and bruiser kill about as quickly as assasins
The issue is that we currently have a damage dealer from every single position while in the past it were usually 2-3 high damage threats per team