Out of all the Eagles, Bernie is hardest to read. It's possible he was asked early on and didn't want to do it. Yes, I can see him turn down the $$.
Don H had probably gotten quite comfortable being 2nd in command of a 4-piece band. Now he's back to a band with 5 "members," including one who is the young and relatively inexperienced son of Glenn, and another quite experienced with a reputation of being a really nice guy; I can't recall reading or hearing anyone speak ill of Vince Gill's character. This will be a different dynamic for Don, who no longer has Glenn to back him up. Adding Bernie, who doesn't seem the type to be pushed around and who is also another guitar player, may complicate things. I would LOVE it if Bernie shows up. But what would he play? I don't think there's even room for Steuart Smith. After all, Deacon will probably play most of Glenn's parts and Vince will probably play Felder's. I think it would be strange to have Vince there with all his skills and not have him play the Hotel California solo with Joe.