Perhaps the issue is that Mark hasn't been the only one working on Ubuntu. Maybe other people who have contributed their time and effort in some form are unhappy to see it go to waste. (You probably don't want those people to go away, by the way.)
Maybe people who haven't contributed anything are unhappy to see a good thing go seemingly bad, as a matter of principle. Or, like someone mentioned, they're sad that Ubuntu's remaining community resources (which might not strictly be Mark's to spend) will now be spent in a way that doesn't benefit them and their friends.
Then there are people who might be offended that they were excluded from Ubuntu's audience in the name of "not excluding people from Ubuntu's audience".