It's nothing to do with the 769s though is it, since by definition they're for different routes. Nor having 769s would place added strain on the diesel fleet, not the electric. More DMUs would be needed, not 323s.
I thought the original plan was to replace 323s with 319 and 769 (and 331 in due course) on various routes? Certainly my understanding was the 323s were due off lease soon. Happy to be corrected if I've misunderstood something. .