I agree about David's choice beeing mature.
But I'm not sure that DS was "an increasingly famous and successful band" when he left it.
I think DS became very big with hits like Romeo and Juliet and later Private Investigations.
At the time of the Making Movies album recording, DS was not THAT big. Of course, Sultans had success, but Communiqué sales weren't at the top, and like Ed says in the Oldifield book, most venues during the 2nd american tour (autumn 79) were half empty.
So in july 80, when David left the band, I don't think it was at such a fame
I think that, DK was mature enough to leave the band.
First, because, in my opinion, he is a "romantic" musician, it means that he had a vision for his career that as apart from MKs vision. So, when Mark came up with a new direction for DS, David's position became weaker than ever.
He saw that his songs will never would be on any DS album or single and his position as a 'rythm guitar player' will be the opposite from his vision as a musician perse.
As a guitar player, DK is not a session/side man. Was not his intent to play guitar as Hal, Jack or Phil.
As Mark, David is an artitist and both has singularities about their artforms. Mark is a genius in his own way, a very smart guy with a very comercial tinus. Mark chance the band directions many times because he knew that if the band had stucked on the style from 77 to 79, they wiould be forgotten.
The problem, in my opinion was that DK, after he left the band, choose another path for his musical solo jorney. I think that he was persuaded to made those strange metal oriented songs and he tooked it to keep a safe distance to his big brother band.
I think that, if DK would followed a path like Tom Fogerty has after his breakup with CCR, is possible that DK had more positive critics about his work. So, in my opinion, DK story could be better.
Proof of that was that, when David releases The Giver, his career, at least from my point of view, starts to be treated with respect.
DK is not a genius as his brother but he is not bad songwriter either. He have good songs and some of them could be part of any Straits record.
Again, if David would choose a folkish style from the beggining of his solo career, his story could be better.
As Hal Lindes said when he was questioned about filling DK shoes as a guitar player from DS: - David shoes are big shoes.