0. This is a case of "really horrible healers". If you happen to have really horrible healers, then your max HP required goes up. If you don't have really horrible healers, what I said sticks.
1. Okay, let's take a real world scenario here, yeah? Let's say you're still a 7,000 HP PLD and you're still fighting Twintania. DS has happened, you've been healed up to full. At no point after this should you be dropping to 1,000 HP. Your healers aren't going to be like "ehhhh one more auto attack won't kill him!". Is this just straight up theorycraft world where the rules of actual encounters don't apply? Where all your HP is specifically to allow the healers to DPS during auto attacks and never ever touch you until the big hits come? Because that's sorta not how that works. Your healer is never going to let you drop low at the expense of DPS, because all that DPS goes to waste if you die and the party wipes. I feel like you have this.. like... concept in your mind that I'm talking about going stupid low on HP and then being like "WELL DA STR DOE???!11". Remember when you were talking about how world first groups want to pump out as much DPS as possible which is why (apparently) the tanks needed to pump out DPS too? Now, consider the following: a lot of the MTs in those groups (most if not all) used i90 Ruby accessories for T10, T11, and potentially even T12. T13 is obviously another story due to a higher HP check, especially during early progression, but eventually most MTs will be swapping to i90 Ruby. Off Tank BiS for early progression was also i90 Ruby accessories for every turn. Why is that? Why don't they wear i110 VIT accessories? Why aren't they using i120 or i130 ones? Why aren't they completely maxed out on HP? Why did everyone migrate to i110 STR accessories in SCoB or Ruby? Are you saying that everyone, but you and this other guy, are wrong? Is the popular method somehow incorrect despite providing results?
2. So, there's a few things wrong with this. Firstly, this is another scenario wherein your healers are bad which will, in fact, bump up the required HP for the fight. That sucks, because it also lowers your groups overall DPS and you're essentially forced to carry someone. If someone makes a mistake, as a tank, it is your job to pop a CD to compensate for the healing, not to just Have Extra Health on the off chance someone fucks up and the stars align so that you are at low HP AND a big attack that could kill you is coming up. Personally, this really really really low percent chance occurrence doesn't factor into how much HP I think I'll need for the encounter. If my healers suck and decide they want to DPS while letting me fall to 1,000 HP and there is some reason that the event you describe can occur, then yes, more HP would be better in that scenario because I'd be at 2,000 HP instead of 1,000 and would survive the... 1,000 damage attack that's coming up. Does this not sound completely ludicrous to you? Is it only me? Are healers out there just super bad or have I been blessed with the most incredible healers in all of Eorzea?
But yeah, if you want extra fluff HP because you're super paranoid that some event may happen wherein someone makes a mistake and this causes you to Outright Die, then that's fine. More HP definitely helps bad players, the Echo buff proves that. Sometimes even Echo can't help bad players. But y'know, if you and your healers are good, then converting that extra HP into STR is a good idea, at least in my opinion. I could be wrong but I reaaaaally don't think this argument of "IF EVERYONES BAD YOU NEED AS MUCH HP AS YOU CAN GET" sways me. I do agree with it, mind you, but that wasn't the scenario I was going for when I gave the T5 example. People can make mistakes, but if both the tank and the healer are competent, neither you nor that person will die. Again, the possibility of you being super low on health (shouldn't happen) + someone making a mistake (can and will happen more often than not) + both healers completely leaving you alone to heal the DPS (extremely unlikely) + the next attack that comes up after this string of events will outright kill you (see: previous parenthesis) + both healers are stuck in their GCD and the attack happens so fast that you die (uh) is such a low percentage that I couldn't give you it if I wanted to. You're clearly convinced that VIT 100% is the way to go, though, so I'll let you keep on trucking with that. If you'd like to get the last word in, feel free to, I'm personally done derailing this topic into STR's weight for a tank.