That worked for the desktop market. But it worked because it was part of a three part strategy (stranglehold on hardware manufacturers, coupled with plug-and-play). The problem is that developer support alone, without the other two prongs, is not a valid strategy.
It might be easier to develop on 7 than elsewhere, but it is all but given that any app that appears on 7 will also appear on Droid and iPhone, regardless of the fact that development might be easier for some developers.
MS’ strategy can work, but only if the novel UI approach is adopted by users. If it isn’t, then the entire strategy crumbles, and they will have to start again.
If the experience of our own in-house developers is anything like typical, Apple is going to have to change it’s cryptic and seemingly schizophrenic approval process before you can say that any app that’s available on WP7 will be available for the iPhone.