CWL and BE will hopefully announce it officially pretty soon to fill those seats up.
Why? The route doesn't commence until September, and will be aimed at business passengers trying to do a day's business in London, Cardiff and the surrounding areas.
Business people tend to plan days and weeks ahead, not months! I would suggest that this could be opened for sale as little as one month before the first flight.
As to whether or not CWL / LCY is viable once the rail infrastructure project is completed is anyone's guess. I would suggest however that when you add security (which will undoubtedly be heightened after yesterday's events) to the mix, along with the location of CWL which is hardly close to Cardiff, I would think that the train will always be the quicker (if not the cheaper) option in the long run.