I think it's a dangerous assumption that they could - because if they could, then they already would have, and really, we'd have about a hundred and sixty immortal humans running around, if not many, many more. A 'diluted line' is a genetic concern for the first two or three generations, yes, but the thing is that you need that 'dilution' in order to create a family line that doesn't fall afoul of its own negatives. If you want a good example of this in modern fiction, look at Charles Stross' 'Merchant Princes' series - which goes south after the second or third book, writing- and concept-wise, but explains pretty solidly why you want an 'inner family' and an 'outer family' which the inner family interbreeds with, exactly in order to prevent those negatives (sickle-cell anemia, anyone) from cropping up.
Only thing is, humans breed like humans - a generation is 20 years. Which means in the ensuing 5,000, virtually everyone in the world is going to be a carrier, and there are going to be a goodly number of actual expressions among them.
Denairastas and the Outcast, I always had the sense, were forbidden - and if the latter continued to 'reinforce' the bloodlines of the former by breeding with them, I think that down the line all the great dragons and other immortals would have come down on the Outcast and Denairastas like an automatic sledgehammer on a Ming vase - complete and utter destruction. Which, IMO, is (or should be) the treatment they receive in SR: 'they don't exist any more'.