The idea that he is the only candidate who can defeat HRC is not entirely fatuous. Even if he fails to gain the nomination (obviously, this is very likely), it could be true.
Trump is viewed unfavorably by almost 2/3 of the American population. His roughly 33% share in the primaries so far is his electoral CEILING, accomplished partly by Democrats voting in Republican primaries to push him above the others. Cruz is brilliant, and a great conservative; I have no doubt that he could make HRC look like a fool in one-on-one debates (assuming they are properly conducted), but he is NOT LIKEABLE, and the Democrats are good at exploiting this. Look what happened to the likeable Mitt Romney - they made him look like an ogre.
Rubio is handsome and charming, and can move to the illusive "middle" quite easily despite his 98% conservative voting record in the Senate.
Americans DO NOT WANT TO ELECT HILLARY CLINTON as their next president. They may want a Democrat, and they may want a woman, but HRC is an odious, un-likeable person with no significant achievements on her record despite decades of opportunities to achieve something noteworthy. The continuing viability of Bernie Sanders is ENTIRELY the result of HRC's shortcomings with the Democrat voters.
American voters will only elect HRC if the Republican running against her is totally unpalatable. Trump or Cruz.
Rubio is the only Republican still standing who could defeat HRC in the general election. The fact that he is doing poorly in Republican primaries does not change this fact.