Meh, if the proprietary software has a good UI, it's going to do well -- open source tends to suck on that front, quite often because it's cross-platform and that doesn't favor the OS X look.

Sure, for programmers and command line users there will be alternatives, but that still leaves a large market.

Y'know what's going to kill Apple indie development in our reality? Apple. A lot of the paid shareware has a long pedigree, and the upcoming de-Carbonization will put an end to many small-shop software.