What is most vital in creating an active market is real competition. (Picture from Vision Connection.)
This is a problem in assistive technology, where we usually have maybe one and one half competitors. That is, Microsoft delivers, and open source tries to catch up. Those are the only two choices. Thus Microsoft is able to use its technology lead to dictate to those needing assistive technologies, then use their needs to dictate to the rest of the market.
Thus it doesn't matter that there are multiple vendors within the Microsoft space, in, say, screen readers. There aren't enough to create a competitive market, because they are all working on the same platform.
If open source could match Microsoft, feature-for-feature, and match Microsoft's delivery schedule, date-for-date, and if open source were also as able to innovate on behalf of its customers as Microsoft has been, then we would have a competitive market, even if Apple never entered it.