By 1980, Zeppelin would have been together as an act 12 years. Bonham and Plant, the youngest in the group, would turn 32 that year. In general terms of productivity, I think looking at the other GOAT rock bands at similar points in their careers probably points the way to guessing the success level they might have experienced continuing forward.
My opinion is that they would turn out to be a spent creative force; at least by comparison to the heights of their earlier works. Its just human nature. They are older. Have other interests. Will have started families. Have money. Have addictions. Will move on to second marriages. Finding time and energy for the creativity that originally made Zeppelin great will be harder and harder to come by. They might break up of their own accord or kick a member out. And there are few musicians or groups who produce equal or better material in their 30s than they do in their 20s.
This isn't to say they won't have successes. But I think looking at Page-Plant-Jones post Zeppelin work shows there wasn't all that much left in the tank. Of course Bonham's death and the end of whatever made their 4 person dynamic so awesome helped shape that to a large degree; however, the dearth of product, let alone much quantity of successful work afterward by any of them seems a pretty big indicator.
According to Page, after
In Through the Out Door was wrapped, he and Bonham were particularly intent on shifting back to a harder sound on the next album. They weren't entirely happy with the softer, more reflective, synth-heavy direction the album had taken.
Guitar World: I thought maybe you were losing your enthusiasm for the band.
Page: Never. Never. In fact, Bonzo [i.e. drummer John Bonham] and I had already started discussing plans for a hard-driving rock album after that. We both felt that In Through the Out Door was a little soft. I was not really very keen on "All My Love". I was a little worried about the chorus. I could just imagine people doing the wave and all of that. And I thought, 'That is not us. That is not us.' In its place it was fine, but I would not have wanted to pursue that direction in the future. (Guitar World interview, January 1998)
How successful they would have been in pulling the band in that direction would have depended on their respective health - Page drifted in and out of heroin habits in those days, and obviously Bonham had become (barely) functioning alcoholic. The
In Through the Out Door sessions were a real change for the band, because Jones became more or less the guiding force at that point - Page was less of a creative contributor than he had been to that point, thanks mainly to the heroin. But since we're assuming Bonham survives - he, at least, would presumably be easing off on the booze. It's also possible that Plant, with more distance from Karac's death, might be more willing to return to a more aggressive direction.
I tend to agree that Zeppelin's creative peak was behind them - 1969-1975, roughly - but that doesn't mean they didn't still have some creative juice left, especially within the synergy of the band. What happens next is that Zeppelin finishes out the European warmup tour, and then does the North American stadiums in 1980-81. The band goes back into the studio - probably both amused and irritated by the explosion in Zeppelin-inspired bands (like Def Leppard and even Van Halen) - and releases a new album (whose exact nature or critical success is hard to make out, but which certainly would "shifted units") in 1982. Another giant tour ensues - they would not have had a problem filling stadiums at that point - and then, given age and the grind of two big tours, the band takes a break, and you don't see another album until 1985 or so. Perhaps they put out another live album to fill the gap - none of them were very happy with
The Song Remains the Same, and they probably would have used the 1981 or 1982-83 tours to obtain better live performances.
Beyond that, it's hard to project. I would say that Plant would have been increasingly keen to do a solo album, as his interests were already diverging even before Bonzo's death. That probably happens in that 1983-84 holiday, and it's not unreasonable to think that Plant's first two albums in OTL provide a rough guide of what it might have been.
If Plant's solo album is successful, it might make him keen to do another, and that might reduce the band's output in the late 80's. It's hard to say.
I would be more hesitant to use the experience of The Who and other big English hard rock bands of the time in trying to determine a direction for Zeppelin. Zeppelin were always operating on a different wavelength, and they had a different internal dynamic.