I am considering to buy a Lumia 950 is it a smart investment considering what is going on with windows mobile platform? since my old windows mobile isn't working anymore.
That's up to how long you want the phone to be current. Chances are updates will stop within the next year. If you don't care about that, or getting new features, and you are happy with the Windows OS, and app store, etc. Then yes, it is a great phone. If you have been thinking of trying something new, then now is a good time to do that also.