Android: not fixing fragmentation after all this time. Essentially, people with less money get less security.
Apple: not making true budget phones with the same lengthy support windows, though this might change as they emphasize services.
Microsoft: leaving the smartphone market entirely. I used to use Windows Phone and it was clearly better than Android. Android was slow and getting updates was a pipe dream. Windows Phone was like a less locked-down iPhone, and around Windows Phone 8.1 the app marketplace wasn’t half bad.
Had Microsoft put out quality flagship phones consistently on a yearly basis on all four major US carriers, they’d still be making smartphones. But people who wanted windows phones were stuck waiting for Microsoft to reorganize Nokia while they mostly crapped out budget phones and had one or two outdated flagship exclusive to a particular carrier.
Windows Phone 10 arrived too late, it wasn’t as good of an update as 8.1, and it arrived after a long drought of phones.