Why a larger torpedo boat was hard to justify

Would a 750-ton naval-treaty limit have been useful for torpedo-boat leaders or large coastal boats?

Only if the navy had a role that justified a new intermediate type; for Britain, destroyers and smaller coastal vessels often covered the likely tasks more economically.

The proposed 750-ton type posed a practical fleet-management problem beyond its treaty displacement. Britain would have had to find a role where it was more useful than a proper destroyer or much smaller motor craft, while paying for another design, training, and logistics system. Smaller vessels could already handle coastal anti-submarine work, weakening the case for the extra category. Institutional cost and role overlap determined whether an intermediate type was worth building, even when treaty rules permitted it.