I don't like to put labels on myself because such words often have many meanings.
I googled what a nihilist is, and I agree with many statements, but not with some, although of course it's not certain that I understood everything correctly.
So, I don't know if my case counts as such.
I live well now — even very well — but I know that in the future I will live much worse.
(And even if that weren't the case, I think I would still choose to CTB. I could probably live the same way I've been living for another 2 to 5 years, just like the past 7, but I still feel like I want to CTB sometime within the next week or month.)
I'm not willing to go through a lot of suffering just for a small chance to get back what I already had.
My reason for CTB is the rationality of this act. I am tired/bored of life, even though I live well. I wouldn't want to live differently (not considering what is physically impossible to get). I think I wouldn't change anything if I could change something in the past. I am afraid of pain and suffering, illness, Alzheimer's, paralysis, strokes, dementia, which can make me suffer and take away control over my actions, taking away the possibility of CTB.
The absence of a global purpose — I see life as a journey from birth to death, and I don't see the point in prolonging the inevitable.
It's like Netflix, which instead of ending a series with a good finale after the first season, extends and worsens it for many seasons ahead while it's still popular, until it's completely ruined and no one cares anymore.
You wrote about happy people — it's not very clear what happiness means. For the last 7 years, I have been doing what I want and what gives me pleasure, 14-15 hours a day(not including sleep), every day. I wouldn't want to live differently — is that happiness? If yes, then I am a happy person. If a happy person is someone who lives so that they don't want to die, then that's not my case.
I think we can't really calculate things like that, because people can often live with this kind of worldview for a long time — until something bad happens that becomes a trigger for action.
And afterward, for others, the reason for their death will be seen not as their worldview, but as that recent negative event (a breakup, the loss of someone close, loss of future plans, job loss, a bad exam grade, etc.).