What Microsoft has said in the past
First of all, Microsoft needs developers to develop for their platforms. One of those platforms is Windows 10. In the past, Microsoft have said that (and I'm paraphrasing) "if an app or service is not there on their platforms, that Microsoft will then be there" I think what is happening is that Spotify has made some commitment to continue to provide their app and service for Windows 10, for a agreed period of time. Microsoft doesn't want to compete with a non-core service. I think this is a smart move by Microsoft but of course, there will always be those who are disappointed by that decision. Microsoft is offering a bridge to the most popular music subscription service, so at least they're not just dropping whatever small number of customers they currently have.