I'm not an expert, so if I'm mistaken, one will surely come along and correct me, but my understanding has been that the issue is not the difficult removal of the parkerizing (it actually wears off rather easily), but the fact that factory-parkerized guns were fitted with much less polishing and surface prep.

As a result, to go from a rougher-surface parkerized finish to a polished commercial blue or nickel is more difficult and labor-intensive.

However, I've come across this discussion primarily relating to military-finished guns which were minimally surface-prepped and received a parkerized or other dull utility finish as a cost-saving measure, like WW II-era Victory or Commando revolvers. For example, S&W would not offer a factory refinish of a Victory to one of the polished finishes post-war for that reason.

I have no first-hand knowledge, so this is just a guess, but I would not think this applies to a peacetime quality gun like an MR 73.