Come on. Let's run down the checklist of fully healthy Miles vs Terrance Williams. Maybe I have it wrong. Keep in mind that it can't be about potential, it has to be at least documented or proven.
Speed: Miles (not even close) 4.38 vs Williams 4.50
Hands: Miles (not even close) catches everywhere
YAC: Miles (1/3 of his 900 yards last year was after the catch)
High pointing: Miles (not even close- see Hands)
Knowledge of offense and rarely out of position: Miles
Strong enough body control to catch in traffic, wrestle ball away, punish defenders, and plank for a TD: not Williams.
Williams as a role player for certain need a tall player packages? Yes
Williams as a #2 WR in place of healthy Miles? Nope