I agree with some of the other comments, there are way too many variables in bumping to worry (or hope) for it. In the overall scheme of things it is still a rare event, right?

That said, I just had my first VDB on UA in years, last night. It was on UX where I had a hunch that it might happen (flight zereod-out, big connection bank, out of a hub). Only after getting to the gate and seeing their pleas for volunteers was it obvious that this could work (I've had many flights where they needed people, only to not need anyone at departure time). Yesterday, in total they had several VDBs and several IDBs.

I had another UX which was zereod-out which I was hoping to get confirmed on due to a change in plans. For weeks it was zereod-out, then a couple days before 1 seat showed up. Guess who took that one? They didn't ask for volunteers, even though it was zeroes again after I switched.

One reason you can have a zeroed out flight and no requests for volunteers is that the agents can see who upline no-shows are and also be able to predict misconnects as close in as 1 hour prior to departure. Just because a pax fails to check in for a confirmed reservation does not mean the inventory opens up in the computer. Another variable is the inevitable weight restriction on a J41.