I don't think it's impossible. It does take consistent investment, and consistently not screwing things up. Between Windows Phone 8.1 and Windows Mobile 10, Microsoft decided that 'one Windows platform' was more important than a dedicated mobile team, and as a result, Windows Mobile 10 arrived several months late and without polish. Their hardware refresh (Lumia 950, Lumia 550, etc) was held up waiting for the software and as a result, they lost momentum. Additionally, promised upgradability from 8.1 devices to Windows 10 came with a lot of caveats.

A great Windows Phone 10 launch would not have put them in the top two, but it may have solidified them as a third platform.