The one button mouse will die when it ceases to be a good option.
You can argue Fitts' Law into the ground, but the overwhelming majority of Windows users I deal with right click once shown how to (literally, as in someone standing over them pointing at the button). Once taught, they point the mouse at that exact place, and right click to get the same menu in the same application for the same reason. They're just following a recipe. If I had a dime for every time I'd done the "right click- no, right- the other button" dance with someone I'd be rich.
Mice point, and click (and scroll, if you feel like killing your wrists even faster). People understand that. Contextual menus require a deeper level of understanding (or a willingness to experiment, which is rare in an age where most people no longer feel like they control their own computers), and most people don't get there. If they have to go there, they follow recipes.
That's not even getting into the crapulent ergonomics of multi button mice.
I'm sorry, but there's no analogy between mice and keyboards, either. The mouse button has an abstract function. The 's' key produces the letter 's'. There are contexts where keystrokes have different meanings (vi, to pick an old favorite of mine, or browsers where the space bar pages through a document), but then there are at least as many contexts where contextual menus have different meanings—that being the whole point. I tend to use ALT-based keyboard combinations in Windows rather than CTRL based ones (ALT-F-S for save rather than CTRL-S) because in my experience the ALT key sequences are more consistent and more reliable. Despite the fact that it involves three whole buttons, typing ALT-F-S is a single motion that I execute unconsciously. I'd have to find the right place to right click to save (which varies from application to application and from state to state within an application), right click, scroll down to save, and select Save. Or I can whip off a quick series of keystrokes with no more thought or effort than it takes to type the word 'and' (less, actually, because it takes two hands to type 'and'). Big difference.
Or, I can go to the highly reliable file menu, find save, and choose it. May be slower, may not be. Depends on the context. What it is, is far more consistently there.
I'm not railing against contextual menus in general. When you have a feature rich application they can pull relevant features out from a dense thicket of options, in much the way that palettes (or Apple's context-sensitive Inspectors) do, and this is an unambiguous good thing. When the task is common enough, though, they're an extra layer of bother, because unless the interface is poorly designed there's a more generic element (especially: generic across applications, so you don't have to think about it) available to accomplish the same task.
It's all about the tool for the job. If I spent a lot of time in X11, I'd run out and buy a three button mouse without hesitation. For gamers, sure, two buttons is not so much valuable as necessary, and if I were a gamer I'd have a two-button scroller. As it is, I have always used a single button mouse with a Mac for the simple reason that it's the best tool for the job. It has almost never gotten in my way, and it has saved my wrists by allowing me to click with two or three fingers instead of one. YMMV, which is why there are so many mice on the market, and which is also why Apple made sure that their OS works with them all out of the box (except the MS mouse, but Apple can't do anything about MS deliberately crippling their hardware...).
So yes, the one button mouse lives. Muahahahahahahahahaha! Even if Apple starts leaning more heavily on contextual clicking I suspect they'll come up with a neater method than two physical buttons for their laptops. Right-clicking on laptops is a pain.
