Add to that the implication in both comments I had yesterday on St Gregory the Great that congregations are aging along with their priests, which means they will inevitably lose mobility and be less able even to commute to a parish not especially distant. But beyond that, as the parishioners age, actuarial reality will catch up, but I don't see those who pass on being replaced.
Nor is this problem new: the loss of Fr Tea at the Anglican Use parish St Mary Las Vegas had exactly the same result. Within five years of its founding, the OCSP has lost roughly 10% of its parishes and groups and is clearly unable to sustain many of the others if any adverse situation comes up. I think a reasonable projection would be that within a fairly short time, it will revert to the roughly half-dozen prosperous parishes that came in as that sort of special case.