But suppose Bob and I agree that AI scientists, or computer scientists, or technology advisors to first-world governments, or some other set of experts, is likely to converge toward the true answer on the feasibility of superhuman AI as time passes, as humanity learns more, etc. Then we can instead make a bet on whether it will be the case, 20 years from now, that a significantly increased or decreased fraction of those experts will think superhuman AI is feasible.

You need stronger assumptions.

For example you need to postulate that the speed of convergence will overcome the natural variability within your time frame.