My view is a bit different. Certainly the PC market was created by multiple software and hardware vendors being able to offer their own distinct take on a universally compatible platform, however in my view times have changed.
For starters, globalisation has meant that proprietary platform providers can achieve sufficient volumes to create plenty of opportunity for developers on the platform, even if overall market share is relatively low.
Second, the PC has always been and remains a somewhat complex environment for the end user. Multiple combinations of hardware and software create choice, but they make for an inherently buggier experience than that which is available in a closed ecosystem. Google is already begining to fight against this fragmentation of their platform and the problem can only get worse. Consumers like simplicity - hence why Mac sales have quietly crept up on Windows PCs in the last 5 years.
Third, web and cloud applications and web browsers mean that closed platforms are really rather open. With arguments about payment, merchandising and advertising headline news it might seem crazy to say that, but in reality 99.9% of developers have no real issue using the closed platforms and benefit from a stable hardware environment.
The future for Apple and RIM on the handset is purely based on the attractiveness of their brands, their hardware and the user experience they offer. If they retain those unique selling points then people won't migrate to Android because it's "open", whatever the penguin-fanciers might think.