Why are DPS so upset about tanks coming close to their DPS?

[Viviza]

Allowing everyone to do decent DPS waters down the role of DPS, because it takes away from their unique niche. Players having a role that the party relies upon them for makes for a stronger feel of teamwork. You don't, for example, see dragoons with a taunt and defensive buffs as a backup tank and doing the job nearly as good as real tanks. In the rare cases that DPS could tank (titan-egi on ramuh, for example) this was patched out quick.

It also makes the balance of the game heavily skewed towards doing DPS and making DPS checks, which in a backwards sort of way lessens the roles of tanks and healers, given they're trying to optimize DPS rather than their own speciality. Right now you have tanks essentially scorning some defensive statistics as a necessary evil, otherwise pushing damage. So you see threads pop up with tanks and healers complaining that their DPS isn't on par, despite that not really being their role in a party. Is it correct that the meta for raiding becomes "push dps at almost all costs"? I don't like it, because it makes a fairly simple system even more focused on that single aspect, but opinions differ.

Edit: I understand that there's an appeal to being able to hit reasonably hard. Tanks and healers who are feeble without a DPS will just push people away from those roles. I'm more pointing out that this comes with some trade-offs.