Jeff: Those artists were on those direct-response TV ads in thr 80s because declining sales cost them their major-label contracts in the 70s.
Mitch didn't have much to do with the jazz artists you mention. Columbia had separate A&R chiefs for pop, jazz and classical. So while Mitch was pushing schmaltz (which I'd argue Mathis rose above, as did Tony Bennett), John Hammond and Goddard Lieberson were responsible for the enduring classics.
Cost-cutting was no doubt part of the motivation in consolidating UNI, Decca and Kapp into MCA Records, but the label made a huge deal in interviews in the trade publications of the time of discussing how they were ditching old-line artists and focusing on contemporary artists.
Essentially, the way it was framed was they were killing Decca and Kapp, voiding remaining artist contracts, moving The Who and Sonny and Cher (as much for Cher's solo stuff as anything) to join Neil Diamond, Olivia Newton-John and Elton John at UNI and then re-naming it MCA Records. The result was one of the few major labels in 1973 not weighed down by a roster of declining MOR artists.