My take on what can be changed for FFXIV

[DrWho2010]

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

Pretty much yes. Again, the amount of people that want to multi job compete after their main is BiS is very small. If they want to BiS other jobs, then they just have to continue playing weekly or wait until the savage tier is unlocked and then they barse all they want later.

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.

It may not a huge undertaking to add, but it's one that is unnecessary to add, nor is it something that is highly sought after. Therefore it is something that resources do not need to be diverted towards. The data the devs have available probably backs up this argument, otherwise this would already be part of the game.