I'm not sure I completely agree on this, and much of what you're mentioning may have been a regional thing.1) What we learned during that period was that the only people who wanted to hear edited versions of long songs like "Beginnings" were people who didn't like those songs and wanted them over quicker. The people who did like them wanted to hear the whole thing.
2) Editing all your records to 3:30 or less---or the majority of them---just meant, to most listeners, that you played more music, but the music was a mix of garbage and butchered great songs.
This became more pronounced as:
1) People began to share more of their listening time between Top 40 and FM album rock stations.
2) Singles sales began to decline (they peaked in 1974 and fell off rapidly after that), meaning that most people who spent their own money on a song were buying the album and expected to hear that version (album sales eclipsed singles sales in 1969 and kept climbing while singles sales went into freefall six years later).
Within three years of Paul Drew's not-quite-ultimatum from the RKO stations, successful Top 40s (like B-100 in San Diego) were saying things like "the whole thing, straight off the album!" as a selling proposition to their audience.
First of all, most people don't buy the record of any given song. Agreed that as the 70s rolled on they were probably more likely to buy the LP as opposed to the 45 but most listeners weren't record collectors. Of my non-music geek friends it was rare for them to buy more than a half dozen records over the course of a year.
Secondly, what you say might apply to material being played on both top 40 and AOR stations, but that was at any given time a third or less of a given top 40's playlist...the majority of what they played didn't get played on the rock stations to begin with. So we're only talking a handful of songs at any given time that were edits with the full versions being played on AOR. And while some edits were kind of hack jobs, that hardly applies to everything.
Again, maybe it was different on the west coast where you live, but in the early-mid 70s AOR wasn't really a mainstream format. It wasn't until '75-ish that rock stations began focusing on the hits and downplayed the left-wing politics that the format took off.