I love a lot of Tommy's stuff. My opinion, totally conjecture:
Tommy wanted nothing to do with Dennis after Kilroy. Ironic that the man who couldn't write about robots could write about a mission to Mars. Though, admittedly it was 30+ years later. Tommy realized that the Styx name was the cash cow. So he "made up" with Dennis and rejoined. JY doesn't care who is in Styx as long as he is but he has more in common musically with Tommy than Dennis.
Tommy bides his time until the time is right to dump Dennis. He's rid of the guy he doesn't like and he gets the Styx name to fill the arenas. It was Tommy or Dennis but Tommy had to get back in to get Dennis out.
I don't like The Mission that much, I think it's medicore. On the other hand I didn't want Styx to become Barry Manilow with harmonies (and I love Barry). I love Girls with Guns and What If. And I love Tommy's Kilroy and prior stuff. I am not anti Tommy. But I think this was the plan all along. Get back in Styx, get JY to agree to boot Dennis and, voila, it's Tommy's band and sold out arenas.
I hope the new Styx CD is great. So far I prefer the two Dennis singles to the two Styx singles.
Some interesting points.
Regarding biding his time, it is interesting to note that the Styx reunion in '96 came right after the Shaw/Blades project went belly up. I really liked the album, but I remember that the planned tour was canceled due to lack of interest. And Ted Nugent had made it clear that he wanted to stick with solo career again, so the Damn Yankees was not an option. So hey why not make up with Dennis. And to Tommy's credit, he took another crack at the solo career with 7 Deadly Zens (an album I still love to this day), but that failed to make much of a dent. And then Damn Yankees attempted a 3rd album in the late 90's, but the magic was not there and they abandoned the idea, so Tommy basically had nada left except Styx, so once Dennis could not tour to being sick (it's not like he just didn't want to do it), Tommy saw his chance to snag control of the Styx name and executed a major power move, which had paid off. Props to him.
As for the Kilroy era, I have always chuckled at revisionist history of that album by Tommy and JY. Check the songs. JY and Tommy wrote the majority of that album, not Dennis.
I am sure the tour wasn't fun, and I totally get that, but for them to slag the album is basically them slagging themselves.
I can't get a lot of love to Tommy's What If album, but Girls with Guns is damn good from start to finish, and Ambition is mostly very good as well, despite the more 80's sounding drum machine bits.