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.