This game is in desperate need of persistent progression content for main jobs...

[Kraniel]

That will always be true as long as people want the hardest content to drop gear upgrade. Imagine if Ultimate/Savage doesn't drop gear but drop cosmetics (whether title, mount, minion, glamour, etc.), but you need to gear up first in order to beat that fight. That would be the only scenario where there is incentive to do something after you gear up your main.

But I assume people don't want to do hard content without being rewarded with actual upgrade to their character, whether in terms of gear or added abilities/traits.

Since that is the case, the incentive then is on your own to make, whether you like clearing the fights again with different people in the party finder or you gear up your alts to clear the fight on your alts or you do other things in the game or unsubscribe for a while until new content arrives.

Again, you're right, but that is not what i was talking about.
I agree with everyone saying that there is/could be more stuff to do after you have geared your main class.

However what i was specifically referring to (and where i agree with OP), was the fact that once you have geared your main class, you don't need to use it anymore for any activity.
Sure, SE can add more mounts/cosmetics in Savage (it's already the case with Extreme primals), but still none of this is tied to your main class' progress. You can farm those any way you like.
For people like myself (and OP, i assume), this is a bit frustrating since we want to put in more hours in our main class, to optimize it as much as possible.

For example, i could totally see some kind of special materia that enhances one of your existing skills when melded on your weapon. A WHM could give Assize an additional HoT effect, for example.
Even crazier, you could get access to a weaker version of a skill from another job.
These are just examples that just came out like this, we don't need to debate if they're a good or bad idea, but my point is : SE has plenty of areas to explore to make the end-game of a specific class longer if they wanted to.

The reason i think they don't want to, is that if they do this, people who play the game more casually will complain that they can not get the same stuff as others, and they don't want to take the risk of losing that part of the player base, which i completely understand.

[linay]

once you have geared your main class, you don't need to use it anymore for any activity.
Sure, there could be mounts/cosmetics in Savage (it's already the case with Extreme primals), but still none of this is tied to your main class' progress.

How is that not related to your main class's progress? You need to be geared to do Savage. So if Savage itself doesn't give gear, but requires gear to complete, then you need to gear up your main to use it to do Savage.