There are just too many examples of political leaders who achieved beneficial results initially but as their reign extended into decades, things went sour. Putin, Mugabe, Erdogan, Ferdinand Marcos, Mubarak, Hugo Chavez, Castro and even the likes of Gaddafi are commonly viewed as having improved conditions for the citizens initially but eventually left a legacy of degraded legal systems, weak civil services, rotten institutions in general, weakened or non-existent independent media and busted economies. It just doesn't work in practice which why nearly all countries have switched to term-limits.