I haven't seen cost figures for awhile, but the addition of the overwing cantilever made the price twice what the single bridge was priced at.

Plus aircraft damage from placing the extra pathway goes up.

Sit at the back of a B737 sometime and time the difference between when the first passenger gets off and when you get off. The difference is only a few minutes. That does not have any help in turning an aircraft faster.

How many times have you waited for departure because the passengers were still boarding? Vast majority of the time the wait is for baggage to be loaded. The cantilever bridge does not speed up baggage loading.