Maybe the bar band Raspberries held to your thought, but at that time when they arrived, they were glitz and schmaltz. Sure I was 17 and filled with my ideas of good and bad in the music industry. But it was easy for me to discern. The Raspberries had their heyday more on top 40 AM Radio. If you were into music, you listened to AM only cause the omnipresence of FM radio wasn't. It was rare to see FM in a car up to 72. So I heard them. My biggest dilemma was 3 dog night, they seemed teeny bopper, but I embarrassingly liked them. Incredible vocals and the live at the Forum is evidence of everything for them. Raspberries didn't come close to to 3 dog night except for lady attraction. Acts in the 70's had corporate influence sure, but they didn't control the Doors, CSN, Zepp,Bowie and most of the bigger name act I cared for like they wood the Rasberries or similar type bubble power pop. Now excuse me,I am going to my corner and receipt some Karen Carpenter lyrics. Yeah yeah Yeah , I know, they wee great. Again, for me I resented this fluff type stuff and comparing it to the more sophisticated music like the Beatles, Kinks, Zepp whatever, their praise was unwarranted in comparison, only fluffed by the corporations or producer moguls. If they were boot strappers as you say, they lost that moniker during their first 5 minutes of fame.No doubt they had teen appeal but so has every other pop/rock act since the genre was invented. It is the music of youth, after all, even if we keep it a part of our lives as we age.
Bubblegum was four or five years old by the time the Raspberries were a national name. But bubblegum, as its name suggests, was premeditated sweetness produced entirely by studio musicians and writers/producers that nobody ever saw. The Raspberries were, quite obviously, a self-contained four-piece including the songwriting i.e. no different to the Beatles who, let's face it, issued more than their share of empty-calorie confectionery in the early days. A guitar/guitar/bass/drums band was a rare bird indeed in the early 70s when they were sharing charts with Olivia Newton-John, ABBA et al.
Without exaggerating, I don't think I've ever seen the 'corporate rock' term applied to the Raspberries. Quite the contrary - they are usually regarded as boot-strappers who came from an obscure NE Ohio town. And if 'corporate rock' is generally accepted as the kind of bands that dominated FM AOR radio (Kansas, Foreigner, Boston et al) then the Raspberries were, by circumstance or design, largely absent from such playlists i.e. their airplay came on AM hit radio. In other words, it can't be corporate rock and bubblegum simultaneously.
That they didn't appeal to you personally does not invalidate their abilities nor their efforts. In fact, the matching suit thing was, in its own way, a revival or a pi** take of the Beatles (them again).
As for 'corporate behavior,' I defy you to name an act in the 70s that was played on the radio that did not go through nearly identical channels: record label(s), pluggers, A&R men, station owners, DJs, etc. It was a closed shop with payola still alive and well and the 'means of production' (studios, record plants) tightly controlled hence the rather inflated cost of vinyl regardless of the year in question. The 'decay and decline' as you term it was the exact same system that also brought most of the bands you name to prominence, artistic originality notwithstanding. I'm sorry but it's a fallacy to attribute ways and means of doing business to a particular band who were simply playing the game under the same rules as nearly everyone else.