It's first and foremost an issue with encounter design.

If fights had a lot more general damage -- enough damage to run healers OOM from non stop max-HPS spam, then the ability to minimize damage as a tank would be more important.

As is, the balance works as such -- I could stack more survivability and...? Or, I could stack more DPS stats and end the fight faster. One has a clear result. The other is situational at best.

If you wanted to fix the dynamic you just design encounters to change that relationship. For example -- I could stack more survivability and actually survive through the incoming damage. Or, I could stack more DPS and be 100% guaranteed to wipe the raid every time due to healers going OOM or just randomly dying to incoming damage.

Then, defensive stats actually become relevant.

Next, you would just have to fix the stats themselves -- so make parry not suck, unlink def / mdef and vit / str stat allocation from ilevels, etc.

You might think this flips the scales in the other direction and makes tank DPS irrelevant but that's simply not true. People will still eventually find the threshold where enough survivability is enough even if it's after you've over-geared content. Also, encounters can still be designed to give OTs the responsibility to DPS. So, offensive stats would still matter.