While you can use voice assistants like Alexa on Windows 10, they still play second fiddle to Cortana. You can't just talk to your computer -- you have to either click a button or use a keyboard shortcut. Thankfully, Microsoft might be a little more egalitarian in the future. Albacore, WalkingCat and others have discovered that Windows 10 test releases may offer deeper support for third-party voice assistants. You could activate apps with a hotword (including when your PC is locked), and possibly "replace" Cortana on a system level. In one test, Microsoft also separated the search text box and the "talk to Cortana" feature on the taskbar.
There's no guarantee that you'll issue Alexa voice commands to your PC from across the room, at least not any time soon. We also wouldn't assume that Google would leap on this given its historical animosity to Microsoft.
A change like this would make sense, however. Microsoft has been warming up to Alexa and Amazon as a whole as it shifts Cortana from direct competition with other voice assistants to a behind-the-scenes technology. This could represent the next logical step, giving you the option of ditching Cortana for all intents and purposes. The company just needs another incentive for you to use Windows 10, even if that means shoving its AI aside in favor of a more popular option.