Mannion wasn't drafted for this year. Hell you don't draft anybody for the present, each draft pick is a long term investment not something to produce right away or be let go. Mannion would have to flop on an astronomical level to not be on this years 53.
While I generally agree with you. this is dependent on position. Last season Donald fell to the Rams, which was amazing, and he contibuted huge. Robinson was called to duty and did OK. Gaines was started and was very good.
Many rookies are chosen for that season and expected to produce. Havenstein better damn well. There is no other choice.
Many positions are expected to do well like RB. Others like WR are not, and yet, sometimes they do.
QB? More rare, but it definitly happens. I agree though, Mannion won't be cut unless he's soooo bad everyone is convinced they shouldn't grab him.