If he "specifically stated" the thing, I'd certainly agree. I don't think any of them did, it was more of an "understanding", I doubt many of them foresaw the concept that a person would want to be in politics for 40 years. The pay at the time was pretty bad, and the job itself was pretty onerous.
I'd guess they viewed Cincinnatus as the model, a person who would "serve" for a time, and then go back to work, as I stated, specifically, earlier.
Why does a wealthy person stay in politics these days? I think it's the status, being recognized as someone important. Most wealthy folks I suspect want the opposite.