A lot of the issues that you raise have less to do with the content itself and more to do with how players network and communities get built. Even before Ultimates were introduced, players would go back to speedrun raids or do minimum ilvl challenges with their friends until the next tier. Even if the content doesn't exist, players will create it as long as there's a sense of community present. Likewise, I'm sure there's plenty of people out there who didn't originally have a goal of raiding but ended up trying it out because their friends were doing it. If your friends are still logging on, then you'll always find something to do.
During the time of ARR, a lot of that networking would have primarily happened within the framework of the game itself. Now, people mostly ignore in-game frameworks like Free Companies and primarily interact with statics and game communities using social media tools like Discord. This is especially important if you're interested in a more niche bit of content, like Deep Dungeons or BLU raids, where you'll find next to nothing formally listed in PF, yet there are lots of runs happening on a daily basis, and new players are generally welcomed. It's a waste of time creating an equivalent in-game social media platform that can compete with this. You'd just be reinventing the wheel.
The downside of this approach is that it leads to communities being fragmented. The game doesn't really need to provide more communication tools, but it does need to play more of an integrative role in bringing these communities together.
I will also say that one big advantage of content like Bozja and Eureka is that it has that 'Barrens chat' effect that results from bringing a lot of players into the same zone at the same time. You end up starting up conversations and friendships by proximity. Interconnected players generally stick around.