No.
You're just trying to outscale dps , which scales faster and harder. If the enemy damage potential is virtually 100% physical damage, there is literally no reason to stop buying armor unless you skill has a health ratio on it or something.
To be clear, the defensive factors with respect to health scale as: linear, multiplicative, multiplicative, multiplicative, and hyperbolic.
The offensive factors with respect to AD scale as: linear, multiplicative, multiplicative, multiplicative, linear proportional, and softly-hyperbolic.
They go tit for tat except AD has 1 more factor even though their 2 factors get destroyed by the defense's 1 factor. The problem is that the hyperbolic factor for defense (aka health regen) isn't even close enough to be a significant factor in increasing survival time, so in almost all cases you discount it as a factor completely. And now offense has 2 more factors than defense (which are BoRK on-hit and ArPen) in addition to going tit for tat on everything else. Except they really don't because the second and third multiplicative factors for defense (AS slow and crit damage reduction) aren't so much stats as singular bonuses whereas the second and third multiplicative factors for offense (crit % and crit damage) are pretty much incremental stats.
So, in conclusion, defense not only has fewer factors that go into maximizing EHP, the factors themselves have a smaller range than the offensive equivalents. Basically, as a tank, your mentality isn't to survive in a teamfight; your mentality should be to survive as long as possible. You are expecting death, and that's fine so long as you did your job to help your team win the fight and then push forward.
That's a broad overview. In practice, it's a good idea to get some EARLY offense in the form of a single item in order to snowball as a tank (usually it is warrior/juggernaut for junglers, BoRK on sponges, and BC/Abyssal on caster bruisers/tanks). And then you go defense.
But again, there is no reason to not buy armor vs an all physical team. That's why you want mixed damage on your team to begin with.
