Why would this humiliate and infuriate Trump and his followers?
If Paul Ryan continued to refuse endorsing Trump and actually saying this was not likely to happen, his actions would emphasize and prolong division of the party. The Bush family and other old guard GOPe would be happy about it. This would affirm they (the Whigs) still held the balance of power, not the (Interlopers aka) Trump Camp. This lack of Republican cohesion would make a Hillary Victory far more likely.