I assume you were being sarcastic when you said that 10-20 million Linux desktops don't count. I'd argue that the numbers are actually larger than that (probably by as much as 2x) but let's stick with a medium number of 15 million... for my discussion below.
Some people want commercial software on Linux, some don't. I attended the Utah Open Source Conference 2009 in Oct. and attended a presentation by a big wig from Adobe where he talked about FLOSS and Adobe. Of course the usual question came up about when will we get Photoshop and various other Adobe products for Linux and the answer was something like, "when there are enough Linux users to guarantee sales of at least 50 million copies". That is a rather high hurdle. Seriously, you have to sell 50 million copies of something before it becomes profitable? What a poorly run company you must have.
But seriously, there are enormous opportunitities in a number of software genres for FLOSS shareware... or FLOSS support funded development. Educational software for K-12, games, small business apps.
The truth is that there is a huge software catalog currently available for Linux any anyone who has been using it knows that. There are some bald spots but those are where many of the opportunities lie.
We believe in the Open Source development model and I also believe in Free Software... and yes there are a number of viable business models available for those groups who can find good business-minded leaders.
Mac hardware sales (especially laptops) have supposedly fluctuated up and down over the last few years. Yes, Mac OS X has poached some Linux users. Anyone who has been to a Linux or FLOSS oriented conference can tell you a significant number of presenters seem to be on Mac laptops... but the good thing about that is how stubborn Apple is. They will only allow Mac OS X to run on their hardware, which is very limited in the variety of hardware components they ship. Mac OS X's growth potential is limited to the number of computers Apple can make and sell. While for a single hardware company they do very well compared to folks like HP and Apple (not talking servers or netbooks of course), a significant portion of their income comes from iPods, iPhones, and the iTunes Music Store. Their concentration on their computers has waned and I don't see that changing unless they finally decide to offer Mac OS X on non-Mac hardware. I don't see that happening... mainly because of the barriers that exist making it work with a larger variety of hardware that exists in the generic PC market. Apple seems to have basically thrown in the towel with regards to servers.
Apple computers are really just another hardware platform targeted by Linux.
I don't really care if Linux ever goes "mainstream". I've been using it as my primary desktop since 1995 and it has gotten so good these days... and as long as it continues to progress is all I care about. I have fears that if Linux ever does go mainstream, I'll have to suffer as it is dumbed down for computer novices... but luckily with the number and variety of Linux distros that are out there, that probably won't be a problem.
I really don't want to have to deal with masses of computer newbies asking me Linux questions and expecting free support... because I'm a Linux guy... and giving free support to friends is what I've been doing for years anyway.
The reality is that 1-5% of the desktop market share actually is a significant number of computers and a large userbase. If Nintendo can make a fortune on the Wii... with about the same userbase as Desktop Linux, why can't software vendors? I realise that with console, hardware diversity doesn't exist and that the hardware diversity that exists in the generic PC market is a barrier for some software (particularly games and multimedia apps) but come on. Many FLOSS projects have accepted the challenge as well as a handful of companies... but it would be great if more commercial FLOSS could fill in the cracks.
Of course the other major barrier that exists is getting more OEMs to offer Linux as an option on more of their hardware... and not to charge more for it.