How do you have a competitive primary with the guarantee of a win?
What I mean is, suppose the primary was competitive, yet you had some magical foreknowledge that she would win in the end.
Or to put it another way, look at it retrospectively: It's June 2016, and Hillary Clinton is the nominee, yet she had to fend off a strong challenger to win the nomination. Are you happy that she won that way, or would you have preferred it if she'd coasted to victory unopposed?
I get that. But it seems to me that the biggest problem with a competitive primary is the possibility of a loss.
There are some other potential disadvantages, of course. She might be forced to take positions that please the base, but could alienate the middle. And an opponent might attack her with something that could be an issue in the General Election.
The Gore campaign criticized Dukakis for allowing weekend furloughs to convicted murderers. Although it's also arguable that this could have allowed Dukakis to prepare for Willie Horton's sudden infamy.