The problem is what happens if an incompetent person stays on forever. There are many examples in history of great countries and empires that were lead by one person only for it all to come down when one idiot became the person in charge. An important idea behind term limits and democracy is sacrificing the ability to have an amazing leader for life so that an incompetent leader doesn't serve for life.

Its a fundamental safeguard and a hedge against tyranny. Its why, in general, countries that are lead by one person for life can be very impressive in the short term but inevitably fall apart once the wrong person eventually takes charge. Because the odds are that eventually the wrong person will take charge.