I'd have to see what this OS as a service means first.

Let's take the generous case: you buy the OS, you get it for life. The subscription simply facilitates updates to the OS, which will be produced on a more regular basis. That can be good for Microsoft and good for consumers. Consumers get a streamlined update process, so it's easier to adopt the latest developments. Microsoft gets a steady revenue stream and can focus upon moving forward with their product rather than supporting old operating systems for a decade.

However, I'm not interested in paying a yearly fee to run an operating system. Application software, I can decide on a product-by-product and year-by-year basis. Operating system software though, I can't. An OS is needed, and the only way to get out is to switch to a competing product. That is non-trivial. On the surface, with an OS that chugs along without a subscription (albeit without new features), that may appear to be okay. Yet there is also the consideration of security updates. That may be a deal-breaker since running an unpatched OS on a public network is a pretty awful idea.

Depending upon the uptake of the subscription services, it may be an issue for Microsoft as well. If uptake is low, Microsoft may be forced to support older revisions for longer than they want to. Only, this time around, it won't be based upon a release that is a few years old or even annual snapshots. It will be based upon rolling releases that may happen several times a year. That doesn't sound fun. In fact, it sounds downright expensive.