For now, does it really matter? Let's see how well W10M will be adopted in the upcoming months. It is doubtful that Microsoft is going to pull the plug before the operating system is even out, so, why worry?
In a year (or 10), if Microsoft decides to do away with W10M, people will adapt, so, sitting here and debating "what if ..." scenarios won't change the way things will be going. Enjoy whatever device you're using. When you're due for a new one, assess the market and pick whatever is available and whatever suits your needs the most. If there will be nothing but iOS and Android, then so be it! These days, it isn't so much about what operating system you're running but if your apps and services are available on them or not.
I don't see the point in this "what if ..." discussions at all!