asking them how common the view in question is at that time and how common it was 20 years earlier, and settle our bet that way.

Planning in 2015 to poll people in 2035 about 2015 seems like clearly inferior to polling them in 2015.

domain experts typically have a general sense of whether some view has become more or less common in their field over time

The hypothesis that this sense is accurate really should be tested. Maybe it's true for things that were controversial at both points in time, but for theories that are eliminated, history is rewritten. For example, the interpretation of quantum mechanics.