I'm quite sure there would be a version of MYOB for Linux if desktop use took off, but I can't see them investing money into it before then - they wouldn't get enough of a return.
My point exactly, they would do it if people are already using linux, but a major barrier to a lot of people using linux is that they haven't done it.
I know hundreds of people personally that would use linux if they could use myob, but without the hassle of virtual machines etc.
If people always accept that it won't be done until more people are using it then it won't get done at all because the 2 largest markets for linux on the desktop are gamers and business, the current linux desktop users are mostly hobbyists as they have to fall back to windows/mac for an accounting system they know how to use or to play a particular game.
There is hope - at least as far as accounting on GNU/Linux is concerned.
Here in .CZ, there is a company making a program called FlexiBee. The program used to be known as WinStrom and as you can see, they've even gone as far as taking the stupid "Win" (along with the implied Winblow$ exclusivity) out of it's name.
It's written in Java and based on PostgreSQL database. The company is based in Plzeň and the sales representative told me that the program is actually developed by quite a few of my former schoolmates (guys from the same college, faculty and major I used to be attending to, many of which like and support libre software as much as we do).
She also told me that their marketing studies noticed increasing numbers of companies running away from Winblow$. Granted, most of them end up as MAC OS zombies (at least for the moment), but I think we should be grateful for anything that keeps shattering M$ monopoly - after all, modifying MAC OS-compatible codebase to add GNU/Linux support is much easier than messing with Winblow$-only bits.
I'm not sure if you're watching the same Google ads as I am (probably not as they seem to be location-aware), but as far as I can tell, the link to it's website actually made it all the way to [Phoronix]'s Google ad box.
Bottom line is that whatever small company wants to start using GNU/Linux now has a chance to do so. And by the way, at least the invoice format is standardized over here (see ISDOC)
In my spare time, I'm administering ICT infrastructure of a small company right next to where I live and over the past few years I gradually got them to a point where 4 out of 6 of their computers are running Ubuntu, so I can tell it's most certainly feasible. It simply boils down to the programs they're using and DOSbox is running their old accounting software just fine. I actually got inspired to try that by seeing a computer in my favorite drug-store booting Gentoo.
However, highly specialized Winblow$ programs (often cobbled together from M$ crapware using Visual Basic and therefore almost impossible to run in Wine) usually make the switch very tricky, especially when they require hardware keys, which are extremely difficult to get working even with virtual machines.