In the case of approaching the haircut due to high demand for HBD, even before existing holders decide to unstake, it is likely that further demand will fall off. Who is going to want to buy HBD knowing that a haircut may be coming? (Of course, we see all sorts of odd market behavior here so nothing can be 100% ruled out.)
As you say, I agree this may just serve as a cap, and it then will be up to stakeholders to decide whether the cap is more helpful or harmful.
FWIW, we have never seen a situation where the haircut/cap was triggered due to high demand for HBD. It has only happened when demand for HBD has been weak but HIVE has fallen even faster.