For ‘bridge authority’ read Renfrewshire Council - it’s their staff. True, it’s not as busy as it once was but with KGV providing a destination and departure point such traffic needs to be managed and unlike many other ports there‘s no ability to allow ships to pass each other until Tail o’ the Bank.
Whether passengers like it or not, river traffic will always take priority until navigation rights are extinguished and I don’t see that happening. The Renfrew and Erskine chain ferries worked because their timetable allowed flexibility (and they could nip behind any approaching vessel as soon as it passed). That’s not easily done with two sides of a swinging bridge.