I agree that it won’t be Trump of Clinton, but Christie or Kasich need a lot to fall the right way for them to win.
Rubio is the default nominee and he’s campaigning like it, not really trying to hard but just waiting to be the guy. The downside to that is that if a credible alternative does emerge, such as Christie or Kasich or Bush, he’s toast. The upside is that he might actually be the only plausible nominee, so why spend all your money?