Let's also remember...Webb is a 23 year old in his 2nd season in the NFL. Even top 5 picks tend to be fairly inconsistent when they're 23 and barely into the NFL. If you're going to try to replace him in the first round...you're likely to continue seeing inconsistent play there for at least a year or two, because that is the norm even for really good rookie LT's.
If you're replacing him, it ought to be a guy who can step in immediately and just own that position for a while. It needs to be a major, FA upgrade. If the Bears are just going to spend like crazy this offseason, that's not a bad move, but that's the level it needs to be at if it's going to happen, and it shouldn't be so much that it prevents the Bears from addressing other positions.
The reason to make that move is that Cutler, Forte, and the defense are ready to win right now, and Webb might still need a couple years. That's got to be the biggest motivation. But that also means that a draft pick at that spot has the same problems...not being ready to win now.