I'm don't mean to suggest that Pld couldn't do the content. Saying it was "damn near unplayable" is probably an overstatement. I did my first Bis Ex clear at roughly item level 175-180, so I know it was doable; however, to say that it was completely undesirable in raiding parties is not a stretch in the least.
You're right to point out that Drk needed to make a strong impression. It was an untested tank, and people were still unsure of it. Even here in the forums, the general opinion about the "best way" to play Drk and manage its skills changed dramatically between 3.0 and 3.2; so, it's not surprising that the first tier of Heavensward Raids was tailored to Drks. That said, needing to make a strong impression for the new job does not mean that they needed to throw an old job under the bus, and that's exactly what they did. Could Pld's clear the content? Sure, but it didn't take very long for people to figure out that they brought absolutely nothing useful to the party and that the meta had shifted from War-Pld to War-Drk.
I wouldn't call that a "liberating moment." Why? Because it would have only been liberating to people who didn't want to play Pld in the first place, and no one ever had to. There was always an alternative to Pld. If you didn't want to play it (even before 3.0) you could always play as a War. Even when Pld was perceived to be the "best" MT, no one was booting Wars out of the party. Pld's, on the other hand, were actively being ostracized just for liking the job. I can only speak for my server, but it was very common for Pld's to be forced to switch jobs or get the boot, and PF's specifically requested Drk's and Wars, or even double Wars. Pld fell by the wayside. It got so bad at one point that I actually ran into people in DF who made comments like, "A PLD! I haven't seen one in months!" Or, "Oh wow! There's still some of you around!" It's not that all those people genuinely wanted to play another job. They could have done that at any time. A lot of them (even now) still want to play as a Pld, but they were being actively punished for doing so.
Worse still, the pre-conceived notions about MT and OT positions were basically cemented in stone tablets and dubbed: The Tank Commandments. War's got so unbelievably shafted down OT pipeline that we'll need an army of plungers to get them out of there. Some of them fell into the bizarre mentality that they're actually a Dps class, and that attitude has persisted to this day (even after their dmg nerf). Thankfully, the Dps checks of the current content has stemmed the flow of the great Pld purge, but both Pld and Drk are still considered MT. Considering the OT position is practically taboo for either of them. Personally, I don't see that as a good thing or a better state of affairs.