Well, only if their developers want to. Microsoft today at its Build Conference revealed plans to get Android and iOS apps onto Windows Phone. Using the new SDK's developers would be able to bring their existing code to Windows 10 with very little change.
Games are a bit easier, iOS and Android developers will be able to port their games to Windows universal apps using the two SDK's (one for Android and other for iOS) announced today. Java / C++ can be used by Android developers while iOS developers can content with their own Objective C code.
Although it might seem as easy as pushing a few buttons to recompile code for Windows 10 but the integration goes far deeper. If say an app uses Google's APIs then it would need to be replaced by a similar API from Microsoft, say switching Google Drive to OneDrive. Microsoft is bullish about this idea, they have already got King, maker of Candy Crush Saga onboard and the game as it exists currently on Windows Phone store is a port of its iOS counterpart with very little modifications. So, Microsoft's underlying work in Windows 10 is paying off.
Microsoft is also welcoming the legacy apps based on Win32 and .Net to come into the fold of Windows Universal apps. For this Microsoft will use its HyperV virtualization tools. Adobe Photoshop Elements will make use of this virtualization technology and is coming to the Windows Store in its Universal app avatar. Moreover, websites will also be able to envelope their apps into Windows Universal apps so as to access system services like notifications and in-app payments without much re-coding or learning new languages.
Now, the ball is in the developers' court as it always has been. Whether to just port their Android, iOS, Win32 and .Net apps and let that be, no matter how inconsistent the UX or dive into Windows Universal apps head-on and embrace some Windows UI elements and Microsoft services. For consumers it means more apps on Windows and for developers its an answer to a question they never asked.
