2004 - Microsoft's Turning Point?
In my experience, for all the achievements of Windows 10, Microsoft and Windows' turning point came around 2004. Mistakes it made then will be nigh on impossible fully to rectify. By early 2004 Redmond had squashed all resistance in desktop OS's, Office suites and, crucially, browsers. The desktop was still dominant - the iPhone was still 3 years away.
At that point, Firefox emerged from the ashes of Netscape. Microsoft could have strangled it at birth but, perhaps because of the transition from Gates to Balmer, Microsoft had its eye off the ball. Just about the only advantage Firefox had over IE6, the then latest version of IE, was that it had tabbed broswing and IE6 didn't. Apart from that, Firefox rendered many websites less well than IE, and was not available to many corporate users. If Microsoft had quickly put tabbed browsing into IE, Firefox might never have got off the ground, and Microsoft could have continued with its work of de-standardising the Internet by deliberately putting quirks into IE which websites would have been forced to accommodate because of IE's 91% market share (according to Net Applications). Over the next few years, the entire Internet might have passed into Microsoft's de-facto control. Websites would have been further optimised for IE, and Mircosoft would have had an advantage in designing server software to cope with the secret quirks put into IE. Apache, Linux and Java might all have been stunted.
Another big trick missed by Microsoft around this time was failure to put Office onto mobiles. If Microsoft had done that in 2004-7, it would have had a fair shot at eating Blackberry's lunch (corporate email would just have used Outlook for mobile), and Microsoft would have had an entrenched position to defend against the iPhone.
Instead, Microsoft waited nearly three years before introducing tabbed browsing with IE7, and chopped and changed its mobile OS while failing to get Office properly onto it. Firefox, then Chrome flourished, and the mobile stage was clear for Apple to take it by storm with the iPhone. From there, Microsoft lost its overwhelming dominance and it is now entirely possible now to build an organisation's IT infrastructure without any use of Microsoft products at all.
Whatever the future holds, it is unlikely to see a return to Microsoft's monopoly of most of the major areas of basic software any more than it is to see a return to dominance by IBM. That may be bad for Microsoft, but being shot of Microsoft's monopolies is surely good for the wider world of IT.