The worst-case scenario would be Jeb Bush (and no other RINOS) against a field of primary candidates who divide the conservative vote.
Consider what happened when Giuliani ran. He wasn't the most wildly implausible presidential candidate ever. But he got just about no support anywhere, because it was felt he'd take votes away from the front-runners, so he pulled out pretty quickly. The voters he's competing for want a winner.
By contrast, conservative candidates linger forever on the merest hope they'd be elected. Their constituents reward purity or consistency or principle and haven't had much of a problem splitting the vote among multiple candidates.
Unless there really isn't more than one or two conservative candidates it will be the same way this time around. Jeb gets in early to head off Mitt, and when Chris enters either he or Jeb will make a quick exit.
And of course, if a conservative does arise who can really challenge Jeb or Chris or Mitt, sooner or later the word gets out that he or she isn't really conservative enough, but that's another story.
Or in many cases, they stay in the race just to keep their message in the limelight, even when they don't have a prayer of getting the nomination. You're right about "moderates" (liberals) in the GOP being more pragmatic about dropping out when their star fades, but then, they don't have a message to keep in the spotlight, they're just mouthpieces for the RNC.