The only noticeable difference is that amateurs lacked the upswing at 50 years, and were relatively more likely to push their predictions beyond 75 years. This does not look like good news for the experts - if their performance can't be distinguished from amateurs, what contribution is their expertise making?

I believe you can put your case even a bit more strongly than this. With this amount of data, the differences you point out are clearly within the range of random fluctuations; the human eye picks them out, but does not see the huge reference class of similarly "different" distributions. I predict with confidence over 95% that a formal statistical analysis would find no difference between the "expert" and "amateur" distributions.