FWIW, I agree that the Borda/Run-off combo is a technically sound solution, but I don’t really like it: it’s too complicated, and, frankly, it’s too democratic.
Why are there so many candidates? There didn’t used to be. The problem that killed the GOP and will kill the Dems is that the party leadership no longer leads. Why isn’t Barack Obama weighing in? What is “unfair” or “inappropriate” about a guy who has won and done the Big Job helping the party understand who can best win and do the job? Why aren’t the Congressional leaders who know these candidates sorting them out, reminding them of their highest use at this stage in their careers, and producing a candidate they all can back?
Where is the bench strength of the Democratic party? Trump, at least, had some rivals — JEB and Kasich come to mind — who would have been decent presidents by current standards. But who do the Democrats have? An old moderate, and old pinko, a few Harpies, and some charisma-free folk who could do the job but cannot win the job. Why is that? Why didn’t “the party” recognize and elevate and train and promote its most competent members?
The solution to the too-many-candidates problem is not a voting system that best decides among them; it’s a political system that precludes their all running at once. A system, say, that would have put forward Hillary in 2008 and Obama in 2016 or even 2020. (BHO is certainly not too old to run for President next year, and he could have done it after years in the Senate and maybe as Governor of Illinois.) But in the age of Twitter, both parties have lost the ability to act strategically as parties.
Maybe the disintermediation of our politics is irreparable, and, if it is, maybe we need to move toward a technical (e.g., Borda/Run-off) solution. But I’m an old guy, and I would really like to see the political parties try once more to act as political parties before surrendering to voting schemes that voters do not comprehend and, ultimately, will blame for any losses in the general election.