If you look through the 10 most recent ArtNews articles announcing new appointments to the position of assistant curator, you get: SMU, WUSTL, UD, UNC, IFA, UW-M and then some contemporary non-PhDs. Those hires probably represents some of the best students at those programs but there are plenty of grads from Princeton and Harvard in dead end curatorial assistant positions at top institutions (the MoMA three-year cap was not put into place to kick out UW-M graduates...)
Of course it's not a pure meritocracy. Timing and luck are oft-overlooked factors... every curator or professor under 45 would say those forces contributed to their success. All this thread fosters is overthinking about the process. If you want to pursue a PhD and you can do it without taking on debt*, go for it. If it leads to a job in the field, great, and if not, you can do something else. It is not that serious.
* A rule without exceptions: no one should take on debt for a graduate degree in the humanities.