How many people are genuinely choosing between a Mac and a Chromebook. In round figured I'd say approximately zero. In which case they're not a threat to Apple.

You do make an interesting point and a good one, but Chromebooks aren't really an OS platform ecosystem. Its more like a feature phone OS for desktops. They're minimal single purpose browser runners.

That's a huge advantage because it's allowed them to completely bypass the incumbent advantages of OSX and Windows, but it makes them a product but not a platform. If they switch to becoming a distinctive platform in their own right then things could get interesting but there's no real sign that they will try that or if it might work.

I am not saying it's impossible for a new platform to establish itself, I'm just saying its a ridiculously hard task to pull off and refuting the idea that a disruption like that is trivial or even inevitable. Just ask IBM (OS/2), BeOS, Sun (NeWS), Microsoft (Win Mobile), Canonical and dozens of others.