They've already done it. They'll do it again. They pulled two MMs for 5E and replaced them with MotM. A bunch of people said "No fair I wanna buy the older versions!" and Beyond just said "Nah mate".
They also - and this is important - pulled the older MMs from Roll 20 and Fantasy Grounds. You can go check. This tells us something about WotC's contract with those companies - WotC can get them to pull material.
The last thing you want to do is have a confused customer buy the "wrong" book, and you have to realize how staggeringly ill-informed a huge percentage of customers are. If you let there be two books both called "The Player's Handbook", you're going to have a huge problem. Some people will buy the old one, and then get mad because it's the wrong one. Doesn't matter if you try to warn them. Some people will buy neither, because they're confused and uncertain (warnings make this much more likely, too). Some people will buy both, then get mad because you don't really need both, and so on. That adds up to a grand total of "more trouble than it's worth".
I will say, looking at this, it actually bodes fairly well for people who think 1D&D will be available on Roll 20 and Fantasy Grounds - I suspect, given WotC got them to pull the older MMs, they'll pull and replace the PHB/DMG/MM for 1D&D too.