This is exactly what they have been doing, and why it costs so much. They have to put a dam in either side of the bridge, dig out a tough for a tube for the hydraulics to run under the canal, then the bridge can be opened from the towpath side by hydraulics.
I think someone at CRT H&S realised a bridge could fall onto a boat/boaters head and CRT might be liable?
Single handed boats (and any crewed boat encountering an already open bridge) go through without the bridge necessarily being secured in the open position (whether by a lock, pole or a crew member). Several bridges, Chisnell in particular, can blow shut in the wrong conditions, possibly helped by being heavy from recent rain?
As far as I know they are hydraulic windlass operated. From the scale of operation at Wolvercote 2 years ago I wouldn't be surprised if it was £150k per bridge😔🤦. They needed permanently running diesel generators powering pumps to shift the water around the two dams, which kept getting over topped by high water levels from rain etc