Microsoft majorly burned their customers. The switch from Windows Mobile 6.5 to Windows Phone 7 completely killed backwards compatibility even though they are based on the same architecture and use the same kernel. The WP7 to WM8 transition completely eliminated any backwards compatibility. WM8 to WM10 was a smaller transition from an architectural standpoint and API but it created a significant compatibility gap that caused several applications to be broken. These transitions were worsened by the lack of upgrades for many devices and high system requirements. These transitions were unnecessary and caused many users to jump ship and caused developers to jump ship due to the extra work required to bridge compatibility with the different platforms. Microsoft screwed up badly.

Windows RT was a mess because it was abandoned with Windows 10 and it was unnecessarily limited to Windows Runtime applications even though Windows RT can br jailbreaked to use Win32 apps.

Microsoft abandoned the Zune Even though the brand could easily be reused. A Zune phone with high storage capacity and slick Zune aesthetics would be an excellent iPhone competitor. Overall the Lumia's didn't have the quality of design the Zunes did.

Windows Mobile pocketpc and Windows CE was an excellent platform which had lots of applications and devices. It was slim and could run much better than Android could on low end hardware. It's similarities to regular Windows made it great for existing developers. It was technically a full Windows system with Win32 and .net though slimmed down.

Pocketpc could have been given a modern revamped interface and used on phones, supporting existing ppc apps with new guidelines for modern apps. Windows CE which is virtually identical to PPC could have been used for arm laptops or for tablets and given a tablet friendly interface.

At every step of the way Microsoft did everything wrong that could be done wrong and I say this as a person that likes Microsoft products.