I've written about this film in a number of online forums and know some of the surviving original members of both bands. I apologize for overlooking this one.
Chicago is a Rock band with horns whose quality fell dramatically after their 8th (or so, some would assert the 10th) album. The band has tried to historically revise itself to be "Terry Kath's" band. This has been done (take this as being acknowledged as polemical short-hand and somewhat oversimplified) to market itself to people who never liked Horn bands in the first place. It was ALWAYS Robert Lamm's and the Horn section's band, albeit with Democratic input from everyone. (The larger issue was one of who made the most money from writing hit songs.) This is my opinion, and I don't think it is much appreciated by Katz, but I saw his TASTE as being his greatest strength. He struck me more as serving the same function as Freddie Green did with Basie's band. Not so much a "I can't wait to show off my chop" guitarist mentality that finds plenty of exposure in the rest of the Rock Music marketplace. It did cause friction within the band as time passed and came to a head as the musically ignorant, highly eccentric underground Rock press "hated on" both bands (and every other idiomatically diverse band with horn sections) pretty much for their entire careers.
People get to like what they like, but B, S&T was an order of magnitude more musically literate in terms of technique and arranging competence and was throughout the first four or so albums. The next two (New Blood and No Sweat) are great records but VERY far removed from the Jazz and 20th Century Classical music that were the inspirations for their founding. (God Bless MAYNARD FERGUSON-Al Kooper's Autobiography). B, S&T's success would've continued were it not for all the "dirty deeds done dirt cheap" by the Nixon Administration. I like Chicago, but by the sixth record, they'd begun transitioning away from their more musically ambitious/diverse roots. There are great tunes and fine performances on many of the later editions of both bands. But the "lack of individual "Hero Worship" that was built into the DNA of both bands (A logo, with the aggregated effort of the entire group prevailing) helped fuel a false narrative that helped drive both Cetera and Kath to feel somewhat disaffected when compared to the "worshipfulness" found in other (less diverse, but equally talented) bands. For most working musicians I know, two contradictory forces are acknowledged: 1. Money and sales figures are a well-acknowledged positive reinforcer. That usually means there's a fairly brutal gap between what you WANT to do and what you HAVE to do. Especially once you become a "Corporate Entity" and payrolls have to be WELL maintained. 2. In the Rock music idiom, the only three things that matter are (to a greater or lesser degree) "Guitar, Bass & Drums".
Exceptions exist, but if people like David Foster felt confident using it against the band by extolling that Cetera (who is unquestionably very talented as a Bassist and Singer) and Kath's inferred unhappiness with his Legacy in general (compared to his peers) then there has to be something self-evident of which to take notice. I.E., the more people (as full-time members and not just session hires) are involved in Commercial Popular Music, the more potential for chaos is injected into their Creative longevity stats. My personal opinion is that musicians and people whose musical tastes transcend the Rock Music idiom liked both bands (and their competitors) a LOT. It shouldn't (and doesn't have to be) "either/or". The ones who don't skip "Free Form Guitar" on CTA and think it's the best track on the set have every right to that opinion. It's just not mine. Either way, (to borrow a sentiment from another of my favorite Artists who listed BASSOON as the Instrument he'd have played if he didn't play Guitar) "America Drinks And Goes Home". Just leave this one corner of Rock to those who prefer it over the many other equally interesting hybrids that emerged as a unique phenomenon. PARTICULARLY at THAT time. Nothing like it has emerged since, and that should be respected. JMO. Everybody gets their own. Best to all.