the trick is to combine all your uncertainty sources together. So in the worst case your numbers still have as much entropy as if you did not have the external source. And even if somebody else knows those numbers they do not know the actual numbers you are using. Of course that raises the question: if your other entropy source is good enough that you are happy in your worst case what is the benefit from some extra source of entropy? So i have argued myself into agreeing with you :) crypto is not a good use case for such a service. The wall of lavalamps mentioned above is a better solution.

Nowadays there are no thermal expansion joints, the rails are welded. Any thunk thunk left is at switches.