I think you've oversimplified the problem with Duvall. First and foremost, the very hot 8 games at the beginning of the season were almost certainly an anomaly, which I think you and others continue to think was the real deal.
Next you point to his 33 dingers in 2016, 31 in 2017, and 35 two years ago (for Miami and Atlanta). What you leave out is that is that, as has been the case this year, Duvall is injury prone. In 2018 he hit just 15 dingers and in 2019 10 and last year 12. He ain't no sure thing this year.
But to me the bigger problem is that Duvall is an outfielder and therefore competing with three of the best bats--all lefties, granted--on the team: Yoshida (OPS .874); Duran (OPS .886); and Verdugo (OPS .817). Duvall's OPS, season to date, is actually a robust .850, but entirely dependent on those 8 games in April. In June his OPS in 18 games was .548 and in July so far (6 games) .617.
That said, I'm fine with not trading for a righty bat unless one is available for low cost, especially in prospects. Keep Duvall, who still could turn things around.
But don't tell Cora he must keep playing him every freaking day. I have defended Cora when he chose not to play Duran, and will do the same for whenever he doesn't play Duvall.