I miss Windows Phone. It ran great on weak hardware, had a lot of stuff built in that Android/iOS dont (mobile IE eliminated the need for email and youtube apps,
OS had a built in QR reader, ece) and it could get updates straight from MS,
along with being able to uninstall carrier apps.
MS could have had a gem if it didnt constantly drop the ball on hardware support/updates. Onedrive integration, continuum, and Domain Management were all talked about for WP, and would have made it a stand out player in the business and prosumer world.
I'd go bigger, and say AMD's utter mismanagement from 2006 to about 2016. A full decade of clowns and morons nearly tanked that entire company. The first phenoms, the delay in phenom IIs, the bulldozer generation, the lackluster 6000s, the ReBrandeon years of the 200 and 300 series, wasting hundreds of millions on seamicro, ece.