Great post, Axmeister.
The only diplodia I was taught about is Pine Tip Diplodia, & yeah it's a pretty generalized description for, "Something is wrong." True to its name, though... interior needle drop is usually a healthy & natural occurance (even when a customer insists the tree has never done that before ? LOL), while tips browning out is a sign of a problem. Like Ax said, non-native species, mature trees, hot / dry summer all contribute.
Sanitation clean-up (get rid of diseased trees / limbs) & a picky pruning (pun, har !).
Diplodia seems to take out entire sections, kinda like how blight spreads, rather that affect the entire tree at once & slowly decline.
I have had success with deepr root ferts (myco, kelp extracts, micronutrients) for slowing down decline.