That is your opinion and you are certainly entitled to that opinion. He was the top rated slot CB in the NFL based on 2018 stats. To me he has a larger upside than any defensive player the Dolphins have on their roster at this time and that includes Howard.
The Dolphins had the worst defense in the NFL last year and I certainly think that if Minkah and Flores could have worked out their differences, he would have been the best player on the defense in 2019.
Minkah was all pro last reason as a safety with the Steelers and the last I checked the Dolphins had zero defensive players on the all pro list. So to me, Minkah was easily the best defensive player on the Dolphins roster when he was traded.