Parmandur
Book-Friend, he/him
Monsters of the Multiverse is a OneD&D product already, yes. They plan to keep selling it for the foreseeable future for the new Core books. Same as everything else they are printing. This is a significant rules revision, but this is Evergreen D&D in action now.But this is also in line with what you do at the end of an edition: you make products that are callbacks to older ones. An anniversary edition (and I have some of those from back in the 2E days on the book shelf behind me) is a limited print run that you intend to sell through.
At the start of an edition, you plan to make new versions of all of the old products to sell again. I had both "Sword and Fist" and "Complete Warrior" back in the day. Now with 5E there aren't that many of those products that even exist. Will WotC want to keep selling Monsters of the Multiverse after 6E launches? I expect that they'd just sell a print run and then come up with an entirely new version. That would be something they could sell to everyone, not just to new DMs. Once a print run sells out, I doubt we'll see new runs of anything as we get closer to launch for the new edition. I suppose it depends on how fast a print run sells, though. I expect smaller print runs and being more conservative until we get an idea of how well the existing edition is still selling. My skills with Amazon searches tell me it is still selling pretty well.