Honestly you can never know, just because one company is dominating another doesn't mean they won't try. RIM (Blackberry) was dominating In smart phones, then Apple decided to get in and they dominated. Now Google (Android) is even getting up there. So again, who knows?
EDIT: Back in 2009 Microsoft said that there will be one, eventually.
I would say there is a good chance they eventually will. Sure Nintendo dominates the portable gaming market, but that is not to say that the PSP didn't have some success.