But who are you worried for?If you have the old book, then sure, of course you can. But will they be keeping the book in print? I doubt it. And that, for me, is where the problem lies: the basic monster information is missing from the core books.
I would assume not yourself as I imagine you already have the 5E14 MM. Future players? Well, if completely new players come to 5E24 D&D and are taught that Orcs use NPC statblocks... then they learn and become comfortable with using NPC statblocks for Orcs and they don't need specific Orc statblocks. They won't have the concern about not having something they never had. So if they won't care that there's no Orc statblock in the 5E24 MM and you can use your 5E14 MM Orc statblock as you might need... then where's the problem? Who's missing out?