Top 5? Brady, Manning, Brees, Romo, Warner, E Manning, Roethlisberger, hell Matt Ryan is well on his way to being better than McNabb is now. McNabb is top 15 AT BEST, and in decline.
McNabb's rating this year is 17th in the NFL. Trent's is 18th. Maybe, a big maybe, a conditional second rounder. But no more. The guy is injury prone, old, and chokes in the clutch.
You are right that he has been pretty good for awhile, but that won't do the Bills any good. McNabb hasn't lasted 16 games since 2003. It's just not worth mortgaging the future.