Minkah hasn't proven he's an elite, or even Pro Bowl, talent in the NFL yet. Last season he played out of position at corner for a few games, and I wasn't particularly impressed otherwise. I was also much lower on him than most when he was coming out because I saw him as a niche player--a guy who excelled at playing in the slot in the flat zone. I view him generally as an average corner, an above-average safety (of which there are many already on the roster), and a quality slot zone corner/safety. As stated, I view Lemarcus Joyner as a better player than Fitzpatrick in just about every respect.
Corner is a much more valuable position and only Conley is the only proven above-average player at the position on the roster. If Conley had indeed gone down, the team would have been in dire straits at corner. Worley has only shown to be average and Mullen is a rookie who got picked on a lot with Conley out. Johnson being good is a complete crap shoot with the odds are that he's not.
I don't see the same think on tape with Howard but I've honestly not studied him very closely.
I think we really only have two solid players at the position if you count Joyner. Worley has played average to below-average through last season, and Mullen and Johnson are rookies with the former getting picked on a bunch last game. Corner is not a position you want to mess around with, and if an elite talent is available you go get him.