My ls ran through some voidwatch for the first time yesterday and we did this NM and something seems to conflict with what you just said. I own an Aegis, and with Shell 5 Norn Arrows was doing about 150~, so to me it looks like magical damage. If you can block with Ochain as well that rapes.
After we finished up with that NM we went ahead and did Hahava. I haven't done the other tier4s yet but I imagine they are somewhat similar. He is a prime example of the mob dictates what tanks you can use. In my mind there is no way any DD tank can do this effectively (atleast @ 90 cap.) Of course, we only attempted with 9 people with 2 of them boxing, I'm sure there are many ways to do this fight if you double your alliance size.
It's really simple when you know you'll need a PLD tank, and that's when a DD tank can't do enough damage to overcome the cures raining down on them. This was the case early on in endgame, where gear options weren't as much, people that were extremely well geared were low, and the playerbase knowledge was low. For example, a SAM couldn't reliably tank Fafnir due to all of the above mentioned, so something like a PLD (or NIN/DRK or RDM/NIN) was used, where they could generate enmity without needing to deal damage ontop of reducing damage better.
So, as long as mobs like that exist, then the need for PLD will be there. In abyssea where HP/curing is infinite, damage spikes are retarded, this obviously isn't the case. PLD still need a bone thrown their way to make sure it stays this way though.