Something that I don't think comes up enough, which is probably because others don't consider as important as I do, but Dennis was a founding member of the band. Dennis and the Panozzo brothers started the band when they were teenagers. Curulewski, JY, and Shaw all came later.
This means that at some point, each of the guitarists joined the band that DDY helped found. Fair or not, I'd think that in the back of Dennis' mind there was always a sense of "hey, you joined this band, so if you don't like it, you can leave". Sure, after years of playing together, and several gold and platinum albums, being a member for a few years longer than the others may not seem like much of a difference to outsiders, but within the band, or at least in Dennis' mind, there was always that distinction. It was more "his band" than Tommy's or JY's. And I would bet that Tommy and JY didn't see it that way. When they chose to move forward without him, maybe they weren't kicking him out as such, but the band had to go on and Dennis wasn't able to continue. Dennis meanwhile saw it as being kicked out of his own band. I've read almost nothing from the Panozzos, ever, so presumably they were both rather quiet guys (at least compared to the "big three") and stayed clear of the drama. But the short version is that Dennis always saw it as his band, and he was ultimately kicked out of it by guys that he had brought in.
I feel certain you're right about DDY's mindset.
I think the Panozzos are actually kind of fascinating and would love to get Chuck's take on some of this stuff (has anyone here read his autobiography?). If you take it to (probably ridiculous) extremes, THEY are the founding members, and DDY joined THEIR band. You could even factor in that Chuck left the band for a year and was replaced by a guy named Tom Nardini, with Chuck switching to bass once he returned. Nardini left, JC joined, followed by JY.
As a fan, I sort of see it as nothing before the first album matters (e.g., I don't consider Jeff Jones to have been a member of Rush), so the initial band was the Panozzos, DDY, JC and JY. Obviously, Tommy is "the new guy" (at least through the first break up).
As with a lot of situations like this, I would LOVE to see how the band is structured legally. The only thing I discern without spending way more time than I have is that, regardless of who wrote the songs, Styx's classic era songs were published by Almo Music Corp. and "Stygian Songs." In 1986, Almo Music Corp. was merged (along with two other corporations) into its corporate parent, Rondo Music International, Inc., which, in 2019, was merged into Universal Music Corp. The conclusion I draw from that is that the record company or management was getting a cut of publishing. Not sure about Stygian Songs. It doesn't have "Inc." in the name, and it was not incorporated (or organized as an LLC) in any of Illinois, California, New York and Delaware.
When you have guys in the band with few or no writing credits (e.g., the Panozzos and Ian Hill), you wonder if they are receiving any publishing income and how much voting power they have in band affairs. At this point, it's obviously Tommy and JY calling the shots, but Chuck presumably still has the same voting power he's always had. Are Gowan, Sucherman and Phillips "hired guns" (a la Scott Travis in Judas Priest) or are they "full members" of the band? Inquiring minds want to know!
Okay, but why would be stop at Paradise Theatre? Mr. Roboto, Don't Let It End and Show Me the Way were all massive hits that helped keep the Styx brand going, so they count.
You're right about the Kilroy songs, and I already made the point that Tommy brought squat to the Kilroy party. Show Me the Way was a hit, but it's not in the "classic rock staple" category, and I was only looking at the "classic era."