If you're doubting McNabb's worth, see the shots allowed heat maps with and without him:
That is worth a 4-year, $2.5M-per extension, IMO.
Along the same lines of him on and off the ice, see LA's shots allowed last year (with McNabb) and this year (without McNabb):
Obviously, there's a whole lot more going on with LA than just the subtraction of McNabb--namely a new coach with a system that encourages more offense--but it's hard to argue, after looking at both sets of visualizations, that LA couldn't really use a defensive defenseman like McNabb right now.
McNabb was underappreciated and discarded by LA, and that was their loss. I'm happy that he's found a home where he and his game are more appreciated.