Maybe not directly but all they have to do is pull the plug on the authentication servers - even singleplayer requires that you log in at least once in order to play (I believe you need an account to play even demo mode) and while that can easily be bypassed without cracking the game* multiplayer would require using a cracked launcher, which is illegal (so is distributing the game unless it were made open source, so no new users would legally be able to play). Of course, they let you switch a server to "offline mode" simply by editing a line in server.properties so you can log in without an authenticated account so there's that (one can only imagine the security issues though, especially on a large server, unless there are plugins for that).
*The game itself does not care if you are logged in or not; in fact, if you play 1.6.4 or earlier versions on the current launcher they do not receive a session ID; the only effect that I noticed was that the Realms button no longer appears, not that it works anyway for any version older than the latest.
But the AFTER EFFECT is what not going to be easily absorbed. Cutting the line is easiest thing a business can do. It's the PR, legal fights (people and even businesses that revolves around the game), and even their rep at large.
Why would anyone wanna pay for a M$ product again knowing that they simply cut a product hot? I don't know about everyone else but I wouldn't go near a company that could one day just "cut me off" of my dependable services (M$ OS versions, software packages, and etc).
Assuming that all Microsoft is doing is discontinuing updates for Minecraft, some people would actually be happy because the modding community for the discontinued Java version would be able to thrive without constantly updating their mods. (Of course, until people inevitably stop joining this tiny modding community.)
Now, from what I've heard, the pocket edition actually has more people playing it than the PC edition. This may not be true, but if it is, that means that a lot of people won't be effected by this change at all.
I'm not sure if Microsoft is legally able to stop people from hosting servers of their game, although it would be pretty pointless...
What about all of the Plugins Developers? Hosting Providers? Configuration Shops? Map Making Shops? and so on?
It not JUST the players get the impact, everyone getting the impact, from the players down to the underlying operations of the JAVA edition.
Shutting it down overnight would be a nightmare. Over months "dying out" sure, but that's assumed with any game. Even major games like WoW will eventually hit "00:00" on their zoom device...
The fear that everyone has is that Microsoft is going to discontinue the Java version, and is going to port the C version to Linux and Mac. (Or worse, they won't, so you'll have to get a Windows 10 PC, iPhone or Console to play Minecraft.)
The PR disaster alone would destroy the rest of the products. Imagine how mad everyone would be if M$ makes Hypixel, Mineplex, and other servers hit the fans.