Problem is there are and have for a long time been tanks capable of greatly boosting their durability to a ridiculous degree.
Like for armor, take Rammus and malphite, Rammus with a maxed W gains a bonus 100%+30 of his total armor and 50% of his magic resistance as much as he wants as the expense of being slowed while its up,
So Rammus easily hits stuff like 400 armor without breaking a sweat.
Malphite´s W maxed grants him 30% bonus armor passively, and its raised to 90% when his shield passive is up.
Like the only real thing that gets through to these guys later on is true damage, max HP damage and a shitton of resistance reductions combined with preferably onhit builds but possibly a full critbuild from someone with a powerful steroid to boost damage.
And as far as magic resistance goes its not so simple,
and
exist which both throw damage out for a loop.
Then finally many tanks can use
, even if they only have like 150-200 armor and magic resistance their health will easily hit anything from 5000 to 10000 k at max build.
Or in some aberrant cases, Cho with his infamous 20 000 health spikes.
Damage is needed, badly needed, because otherwise these guys just walk through your team giving not shits at all just bashing themselves against eachother, it has happened before.
But with that said i do think some baseline increases are in order, champions that are not tanks feel like they ended up more squishy than they should as a result of the increased anti tank stuff.