It increases NIN DPS as well, as with PLD + DRK the NIN would need to keep up DE (and without a NIN you're going to have less tank damage). WAR still does the highest DPS out of the 3 tanks, giving SE to everyone wouldn't step into WAR's "best DPS" territory, but it would allow for PLD + DRK to be a thing without heavy repercussions.
As an example, look at A4S. You've got the boss which only deals magic damage, you've got the add in Quarantine that specifically deals physical, and you've got the Straf Doll that specifically deals physical. PLD + DRK would be a great defensive oriented core here... but since their DPS would plummet without a slashing debuff, you bring DRK + WAR.
Anyway, that's just what I've noticed. I'd personally love all 3 tanks to be equally viable, but I don't think nerfing/changing mitigation is the solution. WAR has to work GCD to GCD to maximize damage and mitigation, and we don't even get the best damage reduction for either physical or magical. We don't need a buff, but we most certainly don't need a nerf.
Something I think that would help, aside from the Eye debuff, is giving DRK a shield equivalent that only works on magic and increasing PLD's DPS to DRK levels. That way DRK and PLD are equal in their respective mitigation departments and DPS. So all tanks could do all content, PLD would excel in physical fight mitigation, DRK would excel in magical fight mitigation, and WAR would be in-between + provide more DPS than the other 2 tanks. Which is how I think SE intended for the jobs to be balanced, and they're nearly there, but the tiny differences in Savage become significantly bigger.
Unfortunately, as nice as this solution would be, that's not how it works. The reason DRK can "replace" MNK is because DRK brings the INT down bit of MNK's DK debuff. The other part of DK is blunt resistance down, which only affects MNK's damage. If PLD had DRG's piercing debuff, you would still want a DRG for Battle Litany and the higher sustained DPS. The only thing you would get with PLD having DRG's piercing debuff is being able to not use a DRG and bring a MNK instead, but you would have lower DPS than just using DRK + DRG.