One cannot insulate oneself from things that are entirely beyond the control of that individual; does not matter if we are talking about the ravages of inflation / monetary policy, militarism, or the fiscal solvency of the healthcare system. One can only choose a specialty, practice, practice mix, and geographic location that affords some semblance of protection and diversification of revenue streams. Dermatology is very pliable in that regard relative to other specialties.
Yes, the politicians screw damn near everything their whoring hands touch.
Regarding the rest: MGMA is a self reported survey, take it for what it is worth. If the discrepancy in pay between single specialty and multispecialty is large for a given volume of work, people will naturally flow to / away from the practice settings that create benefit. If you want another metric, look at the wRVU/comp relationship between different practice settings for same specialty. If you still believe derm is high margin, look at how low the $/wRVU is for derm relative to these "low margin" specialties. The picture is not always as clean as it is sold.