If it were that easy, Chinese pirates would be loading Android OS versions on real BlackBerry devices and not making really cheap knockoffs. They don't, cause the payoff would not be equal to the amount of work involved, if it was even possible.
If people speculate RIM might not have enough to pull off the BB10 launch, what makes u think they'd have enough to launch BB10 and new android based devices?
Thats not a bad idea but it could be confusing. I would say that idea has past their minds. I would imagine, if they decided to release devices with BB OS and others with Android OS, they would just continue to beef up BB 7 and then turn a focus on premium Android devices with a BB style. Creating a new OS and then offering a leading competitive OS could be viewed as admitting defeat.