It's exactly the point at which the loss of revenue due to incompatibility is greater than the cost of developing cross-platform.

Thanks for exactly restating his question. I needed that.

Hey, it made things clearer. For example, when you break it down like that, you might realize that the cost vs. benefit analysis is different for different kinds of software -- there is no "critical level" in general. This is good news for Apple. As their market share grows, software crosses that threshold continuously (as opposed to all at once). That makes their platform become continuously more appealing to users too, creating a positive feedback loop.

Producing cross platform software becomes mandatory for companies that make Windows software currently (unless they don't mind their market shrinking). For Mac only software the market is growing so if they're happy enough at the moment there be even less incentive for them to develop cross platform.

'In the last two years, Mac OS X's share has increased by 3 percentage points, a gain of 58%.'

That's about 25 percent CAGR, so there are still a few years to go before we hit that point, assuming of course that the growth rate is sustainable.