There already are high-end donation perks: custom/enhanced usertitles, custom userimages, and custom ranks.
Name changes can cause a fair amount of confusion, especially during account reviews, promotions, and scam reports. Using scam reports as the example, if a member gets a name change and is not well known, the moderator reviewing the report will only see that the reported user does not exist, and dismiss it. As such, a valid name change requires moderators to be able to keep track of which users have changed their name to what.
Just because a user has donated a fair amount doesn't make them well-known. That easter bunny kid is a perfect example: 100m+ donator, somehow 1k+ posts, and I don't have the slightest clue who he is. Thus, there is a lot of subjectivity in the restrictions for name changes. Who's to say what constitutes a "minor" change, or what makes a user "well known"? Such vague restrictions cannot be made for something that users will need to donate $100+ to get.
Let's be honest. You don't care about other donators: this thread is really an, "I, Spyike, would like a name change: let's make a rule so I can have one." All you really had to do was PM an admin and ask.