Overcrowded races only help incumbents.
I don't think that's necessarily true in run-off states. If Cornyn had been held to under 50%, the runner up would have had some time to raise some additional $$ and consolidate the anti-cornyn vote. The problem is, apparently, there were still enough pro-Cornyn voters out there to put him over the top, regardless of whether he would have had only one opponent or ten.