This is based on the concept of progression clearing content and is a job balance issue and not a playerbase issue.
Players wish to complete content when it's released, it isn't a reasonable expectation to tell players to wait for gear to make their job work in new content, this speaks to class imbalance and bad game design. If one tank can complete content with the available gear when new content is released but another one needs 3 weeks work of tome cap gear (not only on themselves but also other party members) to perform at the same level, that's not a player obsession problem, it's a balance issue.
Players who want to complete content wont wait for broken (under-performing) jobs to be fixed or to take multiple weeks to acquire gear to be on the same level as other jobs when the easiest player-side solution is to either ask them to swap or to find someone else playing those jobs, case and point when it came to PLD and AST having significantly larger issues in progression raid content when compared to WAR/DRK and WHM/SCH.
Players do not have the ability to enact game design changes, they do have the ability to adjust to them however. If something just plain doesn't work or isn't as good as the alternatives chances are players will take the path of least resistance, if you want to fight the uphill battle of class balance then the onus is on you to create a party and try to do so, but you'll have to accept the possibility that players will seek alternatives if they're stopped by the wall of progression due to poor job balance.
You say that you understand DPS checks but it doesn't justify asking a PLD to swap to WAR, the question is if the entire party is putting out their maximum safest amount of DPS and you're still failing the encounter but another tank (WAR/DRK) could safely contribute the extra DPS with no detriment is the party at fault for wanting to seek the winning option?
Sadly the PLD becomes an unfortunate victim of poor class balance (which is a developer issue).