The advantage is that you have a QB you know can play at a high level. Drafting, specifically at QB, is a complete unknown. The ones chirping to draft Lawrence simply hate Prescott and keep repeating drafting Lawrence will save money for the defense.
Here's the issue. The offense is set. We may need a couple players to get the O line better, but overall it's fine, including the QB. Why risk that with an unknown?
Secondly, when the defense is healthy it's better than it's been. It doesn't need a complete rebuild, we can build it further through the draft and free agency at a much lower risk.
The past defensive coordinators and front office haven't focused on the defense. That needs to happen from the very first draft pick. Not after. Even if we trade back for more players.
The problem isn't a need in QB, it's a need to stop the run, pass rush and secondary. Again, the bigger risk is changing from a QB who know is good to one you have no idea but speculation.