Very few people are buying WP7 phones at the moment (even though big manufacturers have put their weight behind it, think HTC, LG). What would make people buy Nokia phones with WP7? Its camera?
Two things:
1) Mango. The WP7 OS actually is probably better than Ice Cream Sandwich, and we'll see with iOS5, but certainly stonger than iOS4. The first rev of WP7 had tons of gaps. They closed pretty much all of them (except tethering) and added stuff others don't have.
2) Nokia still has some of the best phone designers in the world. It's just they had Symbian. Couple WP7/Mango with Nokia and you have fireworks.
This early in the game its dangerous to think you already won.