The current computer based technology was unavailable in the 70s. The way VT is done now was first done by Southern Star in 1993 before that company was bought by Clear Channel.
It could be done, just not quite as simply/easily, when I automated two current hits based stations in the later 70's. Time, correct temperatures and weather, intros, contests, DJ handoffs and all the rest sounded live... or better. The jocks had time to do promotions all day, they did weekend shifts, and did production all in less time than waiting for songs to end.
The system concept has not changed; the hardware just got better.
I should also say that iHeart has invested tens of millions of dollars in new technology. They just bought Triton. They own several companies that provide technical solutions for radio. This is not a company that dislikes technology. It uses technology the way it's supposed to be used. They way we use it every day with personal computers and cell phones.
And that is what quite a few stations were doing with what I call 3rd generation automation in the late 70's. The tape and carts and settings were manually done, and there was a lot of mechanical equipment. But it sounded just as good as today's voice tracking. We did not reduce staff, though. We rededicated hours of staff time to do promotions, school appearances, fund raisers and the like as well as working on commercial production and other things that made the stations sound better.
First generation automation: later 60's based on mechanical sequential systems that simply rotated. Settings done with systems like little "complete the circuit" plugs.
Second generation: electronic based, but still using lots of relays and the like.
Third generation: mid 70's with basic computers and a programming language. Came out the same time as the earliest personal computers and used some technololgy borrowed from minicomputers like IBM's System 32, 33, 34 and others.