If I ran a phone maker, I'd keep a WP7 handset or two in the lineup (because there will be people who'll want it) and because I don't like to be cornered. But I'd also keep a team ready to run with Android without Google support, knowing exactly what I'd have to change if I couldn't use Google brands or software.
But, again, you don't have to depend on Google like you do with Microsoft. And true great devices come from the careful matching of hardware and software and that's hard to do if you don't get the source code.
In any case, I don't see why you think OEMs are particularly dependent on Google. Google has no direct control over Android -- it's just that Google's fork of the Android source code is the most popular with OEMs for the time being. There is nothing stopping OEMs from building their own CyanogenMod-type customizations (after all, Samsung just hired the creator of this ROM) or even forking Android entirely as apparently some Chinese OEMs are doing: http://arstechnica.com/gadgets/news/2010/07/androids-ascent-.... Hell, there's nothing stopping Microsoft OEMs from releasing their own customization of Android with deep integration with XBox Live, Bing search, and Microsoft's other cloud services.