Neither Hitler nor McCarthy were still 'dictating' when they died. Lack of death doesn't mean that everything would remain the same forever.
Stalin's death lead to a good change.
When somebody stay in power for too long it leads to stagnation.
Human civilization has other measures against such stagnation (such as 2 terms limit for presidential position).
However limited life span still adds extra protection against society stagnation.
It's not obvious if benefits of unlimited lifespan outweight potential problems.
I think that doubling life span would probably be beneficial overall, but it's hard to say if further life span increase would.
Neither Hitler nor McCarthy were still 'dictating' when they died. Lack of death doesn't mean that everything would remain the same forever.