Why would they do that? It costs them virtually nothing to simply leave it running and continue to get the business of the users that are left, ditto for the apps that are already on there as lets face it many of the "apps" today are just glorified screen scrapers or simple data processing (such as the Weather.com app which is just grabbing the data and ads from the website) so as long as there are still even a couple hundred thousand users out there using it? its still gonna be profitable with as little as they have to support it.
And considering many places are selling Lumiz 640s for $40 which takes great pictures, gets really great battery life, has a nice clear screen and plays vids well? I can see those being hung onto for awhile. Finally look at how long MSFT has supported their failed OSes, Vista anyone? and it costs a hell of a lot more to support a full desktop OS like Vista than a lightweight version of win 10 which they are supporting anyway. So unless some major malware comes out for it (doubtful as malware writers go to where the most users are so Win10 Mobile is probably the most secure mobile OS there is right now) I wouldn't be surprised if MSFT just left things as they are until the users drop so small there won't be anyone left to care.