returning to the engine issue, won't any "pre-dug-in trench" reduce efficiency and so increase fuel burn?
Probably, I think
LO pointed out earlier, you could open up the tip clearances but sacrifice engine efficiency. The engine casings are the backbone of the engine. If the casings get too flimsy, backbone bending occurs. When that happens, things like third stage fan rotors (blisks) can rub. IMHO, increasing tip clearance is a Band-Aid, there is a more significant problem to be solved.