The guy was having a rough year. McLaren's last title was in 1999, he'd seen his drivers fall out on a scale not seen in F1 for a little while, he thought he'd weathered the SpyGate affair and then Alonso dropped the bombshell.
His drivers, I think at that stage were 1 and 2 in the WDC. It wasnt as if they were fighting a tough battle with Ferrari, like this year for example. At that point in time (I think it was Hungary), McLaren had all titles in the bag. They werent really under pressure.
After the Prost/Senna fall out, Ron Denis should be experienced in dealing with drivers who dont get along. Indeed, it is a sign of a good manager who can get the best out of his employees, even when their personalities clash. Only a bad manager sacks a high performing employee, as it is the easy way out. An even worse manager involves emotions in his professional decisions.
If he hadn't gone to the FIA at that point, it would only have been worse when Alonso inevitably lit the touchpaper.
When Alonso made that threat, he had just got out of his car, after getting pole. He was fully pumped up. He made an idle threat. It wasnt as if Alonso sat down and wrote a carefully worded letter to Ron Denis making the threat. It was a comment made in the spur of the moment.
Ron Denis, for all his years, should've had enough acumen to calm the situation down and read his employee (Alonso), realising that he was just fired up after the qualifying session and what was being said was in the heat of the moment. Ron Denis shouldve thought about his actions before "shopping" his own team as US$100M is a heck of a lot of money.
Even though I'm younger than Denis, I would've sat down with my closest employees/advisers/solicitors and discussed before hand, what the repercusions wouldve been had I "shopped" the team. The guy made an emotional, spare of the moment decision.
In our places of work, we all say things to our colleagues and managers that we later regret. Its all part of work-life. If our managers all decided to sack people and run to the authorities on every little word said in anger, then many people would get sacked and companies would get shut down, due to bad management decisions.