Quote Originally Posted by Kraniel View Post
once you're done gearing up your main class, there is no more incentive to continue playing it.
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.