It's equally impossible for Apple to craft a consumer experience with as wide a reach as Adobe or Microsoft. Apple is not going to get more than 30% market share with their present strategy.
MS/Adobe are unlikely to fall below 60% (especially since this is not a zero-sum game.)
Whether Apple can achieve greater than 30% market share is debatable, not as equally impossible as it is for Adobe to ensure the level of compatibility and legacy support for Flash, and ultimately irrelevant.
Adobe can maintain at least 60% market share - on PC's (Mac + Windows). But they cannot achieve that on mobile devices.
You're ignoring the iPod, the iPhone and the iPad. I might agree that they can't hold onto the huge marketshare numbers they start with as the copycats move in and undercut them, but so what? Then they just go and revolutionize some other market.
MS/Adobe are unlikely to fall below 60% (especially since this is not a zero-sum game.)