This part right here. It's very rare that a QB can come in like Luck/RG3/Wilson and take their team to the playoffs (in fact, it's basically something that never happened until a few years ago with Flacco, Sanchez, and Ryan). If they do, it's generally because they had an amazing team around them or they were just off the charts good.
The Browns are a team hurting all across the roster. A typical quarterback is going to take a few years to develop. At that point, Weeden is already pushing the upper age limit on actually playing in the NFL. Shit, some of that college flash may return at some point, but how old will he be when it does? Would the Browns even be able to get an entire rookie contract out of him?