Skylab B after a working Skylab A is a red herring, since there's no interest for it now that Skylab A did its intended job (Tom Frieling explains that pretty well in the article - zero interest in Skylab replica)
No, the REAL missed opportunity with Skylab B was its possible launch had Skylab A not been repaired; had it been wrecked for good ( for example if the Skylab 2 crewmember - think it was Alan Bean ? - had failed to unstuck the solar array)
Skylab B would have theorically launched 10 month after its failed twin, which place the launch approximatively to March 15, 1974.
The interesting question in this scenario is whether Skylab B, intact and launched nearly a year later, could have waited for the shuttle.
Two interesting side effects of that scenario are
- the lack of Apollo CSM, since Skylab 2 "wasted" a ship to try and repair Skylab A. With only two Apollo launched to Skylab B (suppose the 84 day mission never happens), a lot more oxygen, food and other stuff is left, and in turn, this may increase the chance for a shuttle revisit later.
- Skylab A wreck uncontrolable re-entry sooner or later. This could be pretty bad !
I think this could make a decent alt-history.