I have no idea what happens if a B-Corp gets stripped of its standing, or how easily it can convert to an S-Corp, but from what I understand, the whole point of the B-Corp is to make the scenario you described exceedingly unlikely. In other words, B-Corps exist to free managers from the pressure of these threats exactly, while submitting them to a discipline with demonstrably broad social utility. The assumption is that this is also benefits the kind of patient investors with long-range plans who recognize how counter-productive the market's prevailing short-termism has become. Under the B-Corp paradigm, liberation from this is seen as a competitive advantage.
As long as founders and managers run a profitable company within these parameters, they - and their investors - will be fine. Indeed, they can, in theory, pursue the kinds of opportunities that companies totally beholden to growing their quarterly returns have to pass up. What they don't have to deal with is some short-term "investors" who want to extract a large hit of quick cash before leaving the smouldering ruin of a once-decent brand in their wake.
As long as founders and managers run a profitable company within these parameters, they - and their investors - will be fine. Indeed, they can, in theory, pursue the kinds of opportunities that companies totally beholden to growing their quarterly returns have to pass up. What they don't have to deal with is some short-term "investors" who want to extract a large hit of quick cash before leaving the smouldering ruin of a once-decent brand in their wake.