Seems like you're onto an excellent idea here. Why develop a retro game for current gen platforms when you can develop it for the retro platform directly and emulate on current gen platforms? I wish I could, its probably a lot more fun to do!
Its also quite "easy" to do such games in high level abstracted platforms such as Java or .NET, but that would make it either very difficult or even impossible to get those games to function on other devices such as Apple devices and the like. Emulators are generally available for more platforms than the Java or .NET runtimes. And if you want to make a buck nowadays, you want to target as many platforms as you can :/