I have no idea about sales figures, but my take is that Wipeout HD was undertaken more an experiment rather than as an attempt to make a full-blown game... an experiment that has worked better than anyone could have guessed.
As it stands WO HD does not warrant a full blown release, its far too basic for that, but with several DLC (original content) releases and fixes, it may be.
Sony need all the good games that it can get its hands on, and should support the Wipeout HD fanbase as much as possible.