Your suggestion that a “Top-Two” primary would produce a “moderate,” which you suggest Sen. Flake is, is way off base. If a Top-Two Primary offered general election voters Flake and Kelli Ward, voters would have a choice between two extreme conservatives with no option for social liberalism.
Jeff Flake may be a gentleman, as his speeches suggest, but he is no political moderate, as his voting record shows. On the night of his recent defiant speech on the Senate floor, Flake voted with the Republican majority to undercut consumer protection rules put in place following the 2008 financial meltdown. Earlier in the year, he voted for every ACA repeal bill, no matter how draconian. He voted no on disaster relief for Puerto Rico and no to raise the debt limit for Hurricane Harvey relief. Of course, he voted for Justice Gorsuch. He voted to repeal Labor Department rules requiring the keeping of records on work place injuries, FCC rules on customer privacy, and rules requiring energy companies to disclose payments to foreign governments. According to Five-Thirty-Eight, he votes with the Trump agenda over 90% of the time. https://projects.fivethirtyeight.com/congress-trump-score/jeff-flake/ The NAACP gives him a 25% rating on civil rights and the Human Rights Campaign rates him at 0%.
A race between Ward and Flake would be a choice between midnight black and 1 a.m. black. In other words, it would be no choice at all. If the goal is to engage more voters in the democratic process, the top-two primary is not the way to do it.