I was saying that he would have been our best candidate for the general election back at the beginning of this race... for precisely these reasons.
While Rubio would be far better than Trump, I think he'd be crashing and burning now, much as he did during the primaries. The Democrats would have painted Rubio as a callow and inexperienced pretty boy, just as Trump did, and it would pretty much stick. Rubio never did figure out how to be "Big" in the way that a successful candidate needs to be.
This whole mess points out one of the really serious problems of the existing primary system, as well as the ridiculous "debate" farce. The current system rewards unseriousness. A solid, responsible, statesmanlike candidate simply cannot get traction when the media coverage is focused on who did the best job of insulting everybody else. There's no vehicle for deeper discussion and debate of serious issues: it's all about the sound bites.
Trump's one big advantage in this campaign is that he understood the underlying rules, and knew how to exploit them. How can a Cruz, Rubio, or Walker -- serious men, even if flawed candidates -- compete against a guy who takes nothing seriously, in a system that is itself not serious?