I agree, it's strange that Alan knowingly broke userland, and then argued that he was right. Something more must have been going on.
At the same time, do you really believe that the only way to get people to see the error of their ways is to be as harsh as Linus usually is and to resort to personal attacks (referring to your line about how he could and should have been harsher)? This is indeed Linus's usual style. The reality is that maintaining some basic level of respect with your colleagues would mean not losing great kernel developers to Linus's outbursts. Alan leaving was a big deal and was widely publicized. How many others have left to never return after receiving a nastygram from Linus?
Let's take it further: if your boss publicly called you a "fucking idiot" as Linus is known to do, every time you screwed up, would you want to continue working for her? Chances are, most people would resign on the stop.
Lastly, I disagree. Even if Alan still thought he was right after receiving my version of the email, he would have a much harder time arguing with a more reasoned version. He would certainly be less likely to throw his hands up in the air and leave the team. This whole "Linus needs to be an asshole sociopath to get shit done" is simply not true: most effective project managers don't resort to this type of behavior because they have better ways to get things done. Linus has fallen into this MO, and it's working for him, but I argue that if he was a better manager and a nicer person he would get more stuff done, not less.