This isn't historically accurate. DRK's main advantage over PLD in Heavensward was DPS, and if you played DRK you were expected to be able to swap to PLD as required. PLD was used in a minimum of one world first clear every tier, and there were plenty of fights where it could have gone either way. You were pretty much expected to bring Cover for A2S's Gordian Snipers (Equilibrium was the work-around, but that wasn't universal knowledge). Sheltron was incredible for A3S cleaves. PLD was very nearly used in world progression at a net dps loss just to bring Hallowed to do the Nisi cheese in A4S. A6S strategies were dependent on whether you went PLD/WAR or DRK/WAR, and Tempered was great for mines. A7S was pretty much designed for PLD, and still remains one of the best designed tank fights in this game's history. A8S actually favored PLD/DRK comps in progression due to the Final Beam combo requiring a longer invuln, and Veil was very strong for intermissions. WAR was really the only constant in these fights.

The trade-off in Heavensward was that, if you could get away with bringing a DRK instead of a PLD, your team would be numerically better off for it. But the decision was rarely clear cut during progression, especially for Gordias and Midas. Stormblood simply tipped things such that you always wanted to bring PLD.

I think that the 'utility vs. dps' trade-off in Heavensward wasn't a great one. I think this was exaggerated by the fact that a lot of 'pure' PLD players went from ARR to Heavensward thinking that they could keep being the classic 1-2-3 Shield Oath MT who does callouts on mechanics. If you adapted with the times, there was definitely was a place for your skills, and I've seen some pretty incredible PLD players in HW. But I think Stormblood was the worst for balance as far as tanking goes.