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See my response to Jim Lynch. In a nutshell, and I base this on past history, and also on a view of the components that have changed, there is no way that Ubuntu, or anything else, for that matter, replaces Windows on EXISTING desktop systems.
What I do see is that as new technologies become available, they will replace the desktop as we have known it. We are seeing that happening, with smart phones and tablets edging into an ever increasing percentage of people's computing time, particularly when they are on the move. Some of those people may rarely use Windows any more, but I suspect that most of them still have Windows, either on a desktop or laptop system. For desktops and laptops, I see Windows continue to dominate. If Windows 8 is a bust, people will simply use older versions of Windows, such as Windows 7, or even Windows XP (as millions of desks STILL do, even after a decade!)
Microsoft will, should Windows 8 be a bust, revert to systems more similar to their status quo, continue support of older systems, and/or try again, and that's based on past performance too. Don't expect Canonical and Ubuntu to get more than a few mentions from uber geeks. Most people won't want to go near it, other than extreme enthusiasts who would be looking at something else anyway.
Jim, I wouldn't argue with you that Canonical has worked on, and improved the Unity interface over previous versions. I'd even go so far as to suggest that it could be easier to migrate to than some new Windows interface.
But the question asked here is whether Canonical can actually unseat and upset Microsoft after all these years. To that question, I just do not see how that could possibly happen, and here is why: I just do not see the majority of hardware vendors taking the risk to use any Canonical operating system. Dell has done it to a very limited extent, and they've been on and off with it, with only very limited success. What's going to change to tilt that trend? I see nothing significant enough to move that.
What will actually happen is one of two things: either current customers will, as they've done for a decade already, stick with Windows XP, or they will use Windows 7, which is actually a reasonably good operating system. Canonical, with an extremely heavy push, could pick up a percentage of market share, but I even doubt that. What are they going to add to the equation in any way, shape, or form? Unless they "pull a rabbit out of their hat" that they have not already exposed, I just don't see it, and in fact, they've lost ground in the Linux desktop space rather than gained ground. Linux Mint, which has no pretention at all of "taking over", has taken quite a few Ubuntu "customers" away by actually giving the users what they were looking for. Now that could be one novel way to get a few customers, but no matter what they, or anyone else does, I do not see anyone or anything replacing the EXISTING desktop space.
What I DO see is newer devices replacing Windows altogether. IF Canonical aligns themselves with that kind of thing and bets right on it, they have a small chance to be part of the next move, but right now, I'd say that iOS and Android split that up pretty well.
So my final analysis is that Canonical has very little chance to have a large positive impact in any of these areas; best case for them is to remain a solid niche player.
I think there is a real opportunity here for Canonical, and a risk for Microsoft. Windows 8's new interface is unlike anything Windows users have experienced. For 20 years, the convention has been the "Start" button at the bottom left of the screen as a starting point for most of your desktop activity. Windows 8 changes that, unlike every version of Windows since at least Windows 95, so there will not be the same non-threatening feeling of familiarity. From what I understand, 12.04 is a re-skin of Gnome 3 that incorporates all the biggest changes in the Ubuntu releases since 10.04. Sure it will be different that classic Windows, but since Windows 8 will be too, there is an opportunity here for people to try something new. Plus, with WINE or Codeweaver's Crossover, you can run Windows programs on top of Linux, so there isn't the loss of applications to hold people back. I think that this newest release of Ubuntu will still be a niche OS, but that the niche has a real chance of growing significantly in size. Oh, and Ubuntu 10.04 users will be able to play DVDs for free (take that MS!).