Microsoft’s pitch to developers is to bring their code across without many changes, and then eventually leverage the capabilities of Windows like Cortana, Xbox Live, Holograms, Live Tiles, and more. Microsoft has been testing its new tools with some key developers like King, the maker of Candy Crush Saga, to get games ported across to Windows. Candy Crush Saga as it exists today on Windows Phone has been converted from iOS code using Microsoft’s tools without many modifications.
From developer's perspective difference between "no changes" and "not many changes" is huge (much more than that between "not many changes" and "many changes"). Any change required to provide compatibility with new platform means commitment and additional support costs for developer. Will be curious to see if this helps to bring developers' attention to Windows 10 platform.