Actually, one reason people still use PHP is backward-compatibility. You can take an old project, import it to the latest PHP, and it'll mostly work. Try doing that with node, Rails, Python, or Angular 1.0.
One big mistake Microsoft made was breaking compatibility with VB6 and VB.NET. What happens to all those people who spent years developing VB6 applications?
If you took PHP and refactored everything and cleaned up the bad bits, it'd be a new language and not PHP. You'd be starting over from scratch and competing with all the other hot and trendy languages fighting for users.