Actually Ubuntu already has features for installing third-party packages that could be easily monetised (currently packages have to handle their own licensing sales) the problem there is that it would run into some cultural opposition.
In fact looking at http://www.ubuntu.com/partners/ISV
it appears as though it's already possible for commercial software to be distributed and updated through Software Update; the only thing that's missing is to add a payment method so that sales could be done that way as well.
In fact looking at http://www.ubuntu.com/partners/ISV it appears as though it's already possible for commercial software to be distributed and updated through Software Update; the only thing that's missing is to add a payment method so that sales could be done that way as well.