True...but GnR didn't start out as his side project. A man's got to live...he's left 18 years ago and had to move on with his life/career. But Guns was his true passion.
The operative word being "was". In his book, Duff stated that by 1997, GNR was no longer the most important thing in his life. He also stated the truly magical time in the band was 1985-1988, and that everything got bloated after that.
You said it yourself, Duff left 18 years ago and moved on. His passions (and priorities) are greatly different now. GNR is like an old girlfriend from 25 years ago; the fire just isn't going to reignite, even if people want to keep gazing longingly at photos of them together.
Duff tends to be pretty clear. Such as, he was very clear the last time how it was something of a one off and he was a fill in.
There you go.
You can re-ignite the flame or the passion. I think circumstances dictated his feelings for the old band. Under different circustances I can see him rejoining. Everyone is older and (hopefully) wiser. Also, taking a step away from a situation can help you see things differently. He obviously still has a passion for music.