I think you’ve pretty much hit the nail on the head here, but, after a gathering we had to watch the final episode and a group discussion thereafter, there’s lots to be said about this story.
Without a point-by-point, the one thing that stands out to me is that, once again, one of the major flaws (some people would say strengths) of the new series is exemplified in this episode.
RTD has stated that he doesn’t believe in killing off companions. Fine. The problem is that the new series has redefined the Doctor/Companion relationship. If not downright psuedo-romantic, it is at least a partnership rather than the original mentor/ward arrangement.
With the more parental role of the original series, there is the inevitable moment when the child flies the nest. This is a universal human experience that even the children watching the program can understand.
The new series’ companions fill a different niche – the niche of two people coming together and spending their lives together, in whatever capacity that is, love, friendship, mutual need, etc.
But we also know how that works out – you either live together for the rest of your lives or you break up, often acrimoniously, and if not, through mutual indifference.
Let’s face it, who among us, given the opportunity to travel in the TARDIS, would ever leave voluntarily? They’d have to pry my bloody fingers off the time rotor to get me to leave – even if it was the 7th Doctor.
This has set RTD up with a real poser when it comes time for an actor portraying a companion to quit. The old quick, “I’ve grown up and fallen in love with a minor character in the last episode and have decided to leave” ploy doesn’t fly.
We know Rose would never leave, nor would Donna. Martha’s is the only one that rings true and then only because she’d spent a year of her life away from the Doctor. She was trying to save him and the world longer than she’d actually travelled with him. By his own admission, RTD won’t kill them off, so what can he do?
He dumps Rose in another universe forever. That ending was too sad so he had to bring her back just to give her the “cut-out-and-keep Doctor” (as you so admirably put it.) What’s to do with Donna? What could possibly induce her to go back and be a temp in Chiswick? Guess we have to wipe her mind.
I think RTD is leaving just because he realizes he’s painted himself into a corner and can’t possibly get rid of another companion.