This is not normally a problem at all, IMO. It was not a problem on the Democratic side. It was a problem, in 2016, on the Republican side, but since it was the first time it was a problem in over 40 years, I'm not prepared to say it's a continual problem.
On both sides, almost all of the time, the establishment candidate wins, usually after having absorbed some of the ideas of the more "pure" candidates. That's how our system has generally worked since it's inception, and it's a good system. I don't want to upend it, because if we do we will end up with more Trumps, not less. Make no mistake, Donald Trump is what happens in those rare instances when the establishment loses: you get the crazy.