I don't see why the often used "new viewers" argument would have any meaning. It obviously didn't prevent the 7th>8th regeneration in the TV-movie...
I mean, a regeneration scene would have only needed Paul McGann for a minute or two. How difficult could that possibly have been for "new viewers" to deal with? Some of these "new viewers" might even think that Eccleston's Doctor is the FIRST, since nobody was ever shown to regenerate into him. That's obviously not the true intent...
... or is it?
Funny you mention that. McCoy, himself, has said in interviews that it was a bad idea to have himself involved in the movie for the reasons Davies has stated.
That said, I agree with you 100%. Even just a few minutes of McGann would have bee nice. Show The Doctor escaping the Time War and after a tearful soliloquy, he regenerates. For the new viewers, it would be a nice mystery for them to ponder for the season. Oh, well.