You guys are missing the forest for all the trees. There are 3 components of tanking, and most of you are only considering 2 of them.
1. Establishing/maintaining enmity
2. Mitigating damage
3. Dealing damage.
Contrary to what people seem to think, a good PLD would actually be the better than MNK at #1, if they were better at #2. Sword and Shield, with proper haste buffs/gear (BRD with march+1 hands, haste spell, 25% gear), will actually have shorter attack round intervals than a MNK (PLD: 227 * 0.36 =~82 delay, MNK: 531 * 0.2 = ~107 delay). This leads to it 'refreshing' its VE the more often, and theoretically keeping hate better. However, because having hate leads to the mob hitting you, you lose VE and then take a few hits to knock it back up. However, MNK mitigates damage better (counterstance, dodge), and if it does get hit, it also does much more damage to gain its VE back faster.
So, some people see this and leads to the next logical step, "what can we do to make PLD mitigate damage better?", which is a good start, however, very few continue on to the questions that come after. Say you decide to increase shield block damage reduction and block rate, and tweak it to a rate where PLD can sufficiently keep hate over a MNK. This is the point where a lot of people get off their train of thought and declare PLD as 'fixed', but what exactly does this accomplish? Everyone will still bring a MNK because a MNK will kill the mob twice as fast.
The problems PLD faces are not for any lack of job mechanics, but just being pointless in the current state of game content. With 99% of the game revolving around strictly-timed content like abyssea, salvage, dynamis, or einherjar, people don't have the luxury or taking twice as long to do something. Honestly, the only way I can foresee PLD becoming relevant again is by brute-forcing it through creating mobs with some sort of "only PLD has the slightest chance to survive this" attack/mode, but even then, people will very quickly find exploits and ways around it.