My take on what can be changed for FFXIV

[Shurrikhan]

then there's no reason for the devs to implement a way to let people gear other jobs faster.

By your own claim, at present, the season's progression effectively ends once a player gets their "main" their BiS set.

The rate by which one reaches that first BiS set is unchanged in my suggestion. There is simply means enough to grind out BiS also for other jobs, such that players are not forced to have only one competitive gear class for the whole relevant portion of a tier regardless of their time and willingness to gear up other jobs. There is thereby more available to be done, and more vantage points from which to play the content that would tend to expect competitive item levels.

None of that reduces the time players would stick around. Quite the opposite. Nor is it some huge undertaking to simply add another means of purchase.