[sudosysgen]

While there was large emigration historically out of Cuba of people that disagree with the governments, there were very far from enough people killed during the revolution or later to significantly improve those numbers.

It's not an assumption, there are polls with approval ratings for various political figures from even western institutions that show that approval ratings 40-60 are the norm. It's possible that the approval has decreased due to the crisis, but that's not unique to Cuba and if it is maintained it's plausible for the president to change.