There is a good reason to:
1. Sabo isn’t going to ignore Cobras dying wish. As such his story will involve returning to luffy and it will involve finding Vivi.
Once again, helping ≠ joining. I don’t know why you’re using this as a point.
2. Sabo is literally the Oden parallel of the generation. Hell oda showed that outright wano as such luffy taking the number two of a different top tier to joining his crew at the tail end of the journey to write a book about the adventure would complete the journey
As long as Sabo is still involved with the Revolutionary Army, he wouldn’t be able to join the crew. His dream of being a pirate was something he had during his childhood, but that doesn’t mean he still wants to be one. People can change a lot in 12 years, and so can their goals.
Being the Chief of Staff of the Revolutionary Army is what he wants right now, and he seems perfectly fine with his current role.
I don’t know if the book thing (I’m not 100% sure that I know what you’re talking about. Can you elaborate?) is still relevant either. If it hasn’t been brought up since the flashback then I’d say probably not.
Sabo would be a cool character to have in the crew, but I don’t want him to join if he still has duties to fulfill. It wouldn’t feel right to me.