You clearly haven't wrapped your head around any semblance of what’s actually relevant. Which isn’t anything above lvl70.
Not sure what you mean by not how most people play games. Most anybody (in their right mind) who enjoys RPGs will typically be interested in much of the additional content you mentioned, if not all of it in regards to your sort of completionism. Hence, the main going reason why most people don't play FF14 and some other games that have a similar problem.
A problem that deters most people on a constant basis. Including over and over seeing the same exact story of "I wanted to like this game, but I'm out of here goodbye." That or trying to figure out what is up with this game being so easy, before leaving. And it’s not even users seeking some hardcore play when merely avoiding something this unreasonably easy.
Maybe you’re referring to late gamers who tend to be more vocal as 'most people'. Due to not just putting up with this design, but like it this way. Thereby, having zero regard for anything prior to low and behold - late game. Treating lower level content as nothing other than a hurdle. Maybe with some passing interest in the story and such. But no genuine interest in the actual game itself. Yet late gamers are by far not most people. This goes especially for those who apparently troll the Reddit sub and here as well.
There should definitely never be a real money charge for turning off exp gain. Nor should you have to push an on/off button for it. The fact that you would even need to do so, implies a major problem with the design. Even without the double exp, the amount of exp given by this game at default is not even how any Final Fantasy game in history has ever functioned. While imposing double exp just completely breaks the game for anybody who isn’t trying to bypass content. Which is fine for users who want to do so.
Yes you can start some other classes/jobs. But definitely not an option for users who are only interested in one particular class. Or maybe a few certain ones, and outright don’t want to play any other class.
So yes, there should definitely be leeway in regards to leveling that implies doing side content as part of the leveling process. Basically, things available to do towards a goal of the next level. Getting way too much exp removes any semblance of this. It would make far more sense for to reward much less exp. Therefore, playing additional content implies exp incentive. Therefore, the incentive of exp gain from doing side content is there. Something that doesn't exist in the case of getting so overloaded with exp that you end up leveled too high for even that content.