Windows Phone’s market share peaked at 12 per cent in August 2013, a month before Microsoft’s acquisition of the phones unit was announced.
That 12 % is cherry-picked from sales in particular markets. Worldwide and Windows Phone has never been above 5 %, which is why Nokia thew the towel in.
The numbers quoted about the most popular Lumias would appear to back this up: people are either sticking with what they've got or are moving to Android or IOS. You might expect the typical two-year contract and phone renewal to work in Microsoft's favour: switch to new phone with new OS (Windows 10). But it obviously isn't. Here the lack of compelling new phones, no doubt due to pink slips and lack of investment since the takeover, is going to cause problems.