The big MSOs were either involved in networks or TV (or both). Bonneville owned radio stations and did format syndication.
But Bonneville did, essentially, one format: Beautiful Music. They did not run it as a network, and did not do sales representation for the subscribers.
What started to happen in the 70s was the larger radio-only MSOs appointed group PDs in order to better merchandise their reach, So a company like Sonderling or Metromedia might not own a network, but they could sell their major market reach, rather than strictly selling local markets. That was when radio as a local sales tool started to change.
Even back in the Storz and McLendon era of the 50's and 60's, the group owners had national PDs or the equivalent.
In 1970 when I managed one of the Mooney stations... a group in just 4 markets at the time... Joe Sullivan was National PD, and when Joe left we got Scott Shannon. Of course, they both left my market alone.
The breakdown of the 7/7/7 model was caused, in my opinion, by Docket 80-90 in 1983, when the FCC started adding more stations to markets, driving down market share.
But Docket 80-90 did not have major impact until the end of that decade. 80-90 was the product of the Bonita Springs case where a Class A tried to become a Class C, provoking counter filings. They FCC decided that "major applications" could not open a filing window and that expanded to allowing other major moves like creeping into bigger markets and an amendment of the table of allocations.
The biggest impact was in 1989-1990 when all that came to fruition.
That was what caused a number of heritage radio owners to sell their stations in the late 80s & early 90s. Concern over who would then own radio stations led to the FCC loosening ownership rules several times during that time, before ultimately changing the rules entirely in 1996. But anyone who was working for one of the bigger companies in the early 90s could see that changes in technology were going to make national syndication easier and cheaper.
The main reason certain owners were selling was the inability to own more stations. 7/7 was just too small for a company to even have a radio division.
In the 80's we had plenty of national syndication as satellite became viable. The thousands of stations that had used taped services like RPM, Peters, Drake-Chenault, RPM, Kala, SRP, Bonneville, Kala and others began taking programming from SMN and others. Satellite was a lot more flexible and allowed unmanned station operation with no tapes to change, so there were huge economies.