I still don't think that him captaining the Stargazer for twenty years is enough to surmise he'll command the Enterprise-D for twenty years - the length of time he captained the Stargazer is a single data point which you're extrapolating out from.
I get that, & opinions are what they are, but I don't really see it as a data point. I'm not sure the 10 year old car analogy quite fits either imho. It's a life choice, a career attribute, a personality trait.
Look at it like this. I personally have had the same job, in the same place, for 20 years myself. If I went out tomorrow to get another job just like this one, that longevity would certainly be worth recognizing. It represents reliability, even if it's only one example. How many 20 year examples in a person's life is there going to be after all?
So back to Riker. If I'm him, & I'm shaping my future. Do I take the gamble that I have an off chance of snagging an assignment, in the near future, which is occupied by a guy who's most well known other assignment went for 22 years, or do I assume that he quite possibly could be a lock here, & maybe I ought to be looking around for other opportunities, unless I want a potentially loooong wait, only to be disappointed anyhow, because I might have squandered any chance I had to distinguish myself otherwise, while waiting for him? What's the smarter course to pursue really? He's been refusing the leadership role the whole time he waits
After all, it's not as though
Captain Riker of the USS Melbourne has any
less of a chance to become the new 1701-D's captain, if he
wasn't there when Picard left anyhow. Refusing promotion, while waiting around for his "Crack at that chair" is just not a good life plan. It actually diminishes your stock in the eyes of those who'd promote you. At best you appear diffident, & at worst entitled, or selfish