OK - the argument, as I understand it, for drafting a rookie QB is that you get them under a rookie pay scale for 4 or 5 years, depending on the contract. that, in turn, means theoretically that the team has more money available to sign other players and put together the best possible team. once the rookie QB goes off the rookie pay scale, that's when you run into cap issues.
but - If you are paying a veteran QB like Cousins a market-rate salary for 2 or three years, you essentially lose the advantage of a rookie pay-scale QB for those 2 or 3 years, leaving only 1 or two years to capitalize on the benefit of a lower salary QB.
hence the idea that, if you draft a rookie like Penix - you do not sign Cousins to a market-rate deal. you go out and sign a lower-cost veteran to be a bridge QB for one year as an insurance policy. an extra $20-million in cap space could mean that you re-sign one or two veterans players who can help the team win.
I am NOT saying Cousins is a bad QB.
I AM saying that teams have to make choices, and I would rather pay a rookie QB and re-sign other players as opposed to having Cousins eat up $40-million+ of the salary cap.