First of all, the Bills could have easily retained all of those players, at the expense of signing no one else. That would have been incredibly dumb. McGahee was a goner and a waste of space. Fletcher didn't fit the scheme. Spikes couldn't be counted-on to return to his pre-injury form. and Holcomb would have been 3rd string next year.
As for Clements, it would have been nice to keep him. But for $80M, I shudder at the thought of him having a year like 2005, or the first half of 2006. Again the Bills could have easily outbid other teams for him, but decided to let him go.