So trading a 2nd round pick for a back up is a good move in your opinion?
Yes. Because Rosen ending up a backup is the 2nd worst case outcome. If we really do pick at the top of the draft, pick a QB, AND that player beats Rosen out, then we likely get back more than a late 2nd next year.
It's a option contact with the possible outcomes (ignoring injury possibilities):
* He's so good, we don't draft a QB (even with #1) next year -> We either get to trade out or land a blue chipper at another position
* He's good enough, but we still draft a QB next year -> More likely than not, we get more than our late 2nd invested on the looser of the playoff between the 2
* He's not a franchise starter (for us), but he might have significant value for someone else -> More likely than not, we get at least the late 2nd we invested
* He's not a franchise starter (period), but we still can get some value (more than a late 2nd) because of another team suffers an injury just at the right time
* He's a good backup -> The 2nd we invested in him starts to look pricy
* He's just not an NFL player -> we rolled the dice and lost