Even if such ennui is “natural” (and I don’t see how a phenomenon that only shows up after 5-6x standard lifespan, assuming parity with baseline humans can ever be considered natural), it should still be considered a problem in need of solving. And the Culture can solve just about every plausibly solvable problem in the universe!
Think of it this way, if a mid-life crisis reliably convinced a >10% fraction of the population to kill themselves at the age of 40, with the rest living happily to 80+, we’d be throwing tens of billions at a pharmacological cure. It’s even worse, relatively and absolutely, for the Culture, as their humans can easily live nigh-indefinitely.
Even if you are highly committed to some kind of worship of minimalism or parsimony, despite infinite resources, or believe that people have the right to self-termination, then at least try and convince them to make mind backups that can be put into long-term storage. That is subjectively equivalent to death without the same… finality.
This doesn’t have to be coercive, but the Culture demonstrates the ability to produce incredibly amounts of propaganda on demand. As far as I’m concerned, if the majority of the population is killing itself after a mere ~0.000..% of their theoretical life expectancy, my civilization is suffering from a condition that ours standard depression or cancer to shame. And they can trivially solve it, they have incredibly powerful tools that can edit brains/minds to arbitrary precision. They just… don’t.
I find it very hard to believe that a civilization with as much utter dominion over physics, chemistry and biology as the Culture would find this a particularly difficult challenge.
The crudest option would be something like wiping memories, or synthesizing drugs that re-induce a sense of wonder or curiosity about the world (similar to MDMA). The Culture is practically obsessed with psychoactive substances, most citizens have internal drug glands.
At the very least, people should be strongly encouraged to have a mind upload put into cold storage, pending ascendance to the Sublime. That has no downsides I can see, since a brain emulation that isn’t actively running is no subjectively different from death. It should be standard practice, not a rarity.
Even if treated purely as a speculation about the “human” psyche, the Culture almost certainly has all the tools required to address the issue, if they even consider it an issue. That is the crux of my dissatisfaction, it’s as insane as a post-scarcity civilization deciding not to treat heart disease or cancer.