I would venture to guess the average MySpace user has no idea about the people they have fired. They'd certainly notice if something happened to Tom though.
Tom Anderson doesn't even run the "Tom" account. In fact, Tom's entire identity on the site is mostly fictitious, complete with a falsified age. He's pretty much a figurehead, all they're paying him for is to avoid making any negative press about "MySpace fires Tom" or something like that.
You seem to be correct, they can't replace Tom as the default friend. They'll have to do something really clever for that to happen ...
The other day I was talking to my friend and she was telling me that facebook suck's because it does not support layout changes and allows auto music play as soon as you load the page. So, in the process I told her about the layoffs and Facebook has crossed over Myspace in terms of monthly visits... she was totally surprised.