What will developers do? WP7.8 or WP8?

I know the common wisdom is that with Windows Phone 8 apps not working on Windows Phone 7.8 that old devices are being abandoned. In the long term that is certainly true, but is it in reality. Other than some showcase apps that leverage the new hardware features of WP8 will phone app developers move to WP8 quickly? Or will developers choose in 2013 to move slowly until enough Windows Phone 8 devices are in use and people have moved on from Windows Phone 7 devices. I think most of them will take the slow route since all the WP7 apps will work on WP8 why hurry? Now apps that are logical to run on a Win 8 or Win RT tablet will move over more quickly.