He may well decide to run independent. "I have no plans" isn't the same thing as "I'm not going to". And I have never harbored any illusions that he'd be the Republican candidate. Even if he somehow did win, he wouldn't win. The RNC and media would simply declare Romney the victor by fiat (which they have already done).
If Paul doesn't run independent, I'll probably vote for Johnson. I can't in good conscience vote for Obama, but I consider Romney more of a threat and I'll tell you why: Yes Obama is more liberal, but Congress won't let him do anything. They will let Romney run wild out of "party unity" just like they did with Bush. Not only is that bad for the country, but it's worse for the Republican party and the efforts to reform it. An Obama Presidency would be 4 years with an inept lame duck. A Romney Presidency would be a complete disaster.
Just my opinion, of course