I appreciate David Mermin’s letter. There is, of course, an ongoing debate in the philosophy of probability about whether probability is best thought of as objective or subjective. Experts continue to disagree, which suggests that we might want to acknowledge a “problem” in that wider-than-quantum context, even if we think we have the right answer.
Whatever our stance is toward probability, we can still wonder about reality (ontology). People disagree about that too, and there are respectable but mutually incompatible possibilities—which represents another problem.
In particular, one can be a subjectivist about probability (as I am myself!) within different approaches to the quantum measurement problem. Both Everettian and Bohmian quantum theories invoke subjective probabilities, concerning which branch of the wavefunction you are on (Everettian) or the values of the hidden variables (Bohmian), although they treat quantum measurements differently. Taking a subjective stance toward probability does not by itself resolve the measurement problem.
Objective-collapse models, by contrast, sit more comfortably in (as the name suggests) an objective picture of probability. To the extent that such models are empirically viable, it seems wrong to deny the existence of the quantum measurement problem, since, again, they address it very differently than other models.
I am optimistic that the quantum measurement problem is solvable and will be solved, but I am wary about prematurely declaring victory.
