Like Bert said, the firm was doing fine without Don. Don was pretty deluded in whole heartedly believing the praise people heaped on him all those years, which led to his abusing many people's trust. Yes, he's a massive talent but he isn't indispensible. The world will keep turning without him. With him being on probation, it was pretty obnoxious for him to swing into Bert's office with the new business idea (particularly for a puny company). Bert, Joan, Peggy saw through his bs. Maybe now after his bender he'll be sincere in his actions.
No one is indispensable; that's one of the major rules of business. However, "fine" is not "outstanding" or "stellar." If the firm was doing either of those they could have bought out Don's partnership shares and had done with him. He's also still a partner so he was well within his rights to go see Bert. In fact, the reason he went to talk to Bert, was so he could follow the rules and hand off the "virgin." No doubt landing an expanding company in an expanding market where the market itself is not advertising, and the firm in question is a "virgin," is a major find. Don was being a team player and whomever he handed off to had stood to earn a major score. Bert was being stupid, and also being out of character, as Bert is not stupid. Think back to Bert mandating Don not fire Peter because of his family connections even though Bert (and everyone else) loathed Peter.
No, Bert was completely out of character.