Given the more collaborative nature of NMB (and FC), I think Neal was just trying to get some more solo music out of his system before committing to regrouping with any of his other bands. I think there's also been enough time since the last FC album that the other writers in the group have probably enough material to bring to the table for a new album, and I've been hoping they get back together for a couple years now.
As for NMB, I think Eric and Bill (and possibly Randy too) also have contributed a lot of material over their last four albums together, and probably more and more with each album, so I'm sure they've got material saved up for NMB5 as well, which I'm sure is a blessing for Neal, who might not have 50-100 minutes of prog music demoed for them again.
If I had my way, I'd love to see Neal and Mike bring back Flying Colors later this year to write and record their fourth album, release it next spring and do a small summer tour. Then with the NMB, they could write and record NMB5 in the winter before FC4 comes out, then release it in the summer or fall, and then do a fall/winter tour with Morsefest 2024 beforehand.
Unless Neal's age and health are catching up to him, I doubt he'll retire anytime soon. He's still full of energy and ideas and if touring is wearing him down, he may become a strictly-studio musician from then on, which I would be OK with. I'd love to see him do one more album-tour cycle with both NMB and FC though, so I can, hopefully, get a chance to see them live before they're disbanded. I missed my chance with Transatlantic, but now that I have a better paying position at work, I'm hoping I can try and financially fit in a concert to see NMB and/or FC if/when they tour again!
-Marc.