Why do long-term great leaders inevitably lose their grip, then need replacing ASAP?

My understanding is that we can perhaps see (currently) an example of the only great national leader of this world era having lost-grip; such that removal and replacement (which will, inevitably, be with someone less great, because greatness is so very rare) is apparently both necessary and indeed inevitable. 

I have observed the same in sport; which is one of the last remaining aspects of Western life in which good, and sometimes great, leadership can be observed. My sport is cricket, especially the England team; and I have seen two outstanding England coaches - Duncan Fletcher and Andy Flower (both, interestingly, born in Rhodesia). 

Both took England to the rare pinnacle of winning the Ashes, and Flower managed this more than once, including in Australia. 

But both "went off" at the end of their tenure; making strange and bad errors of player selection and strategy - strange, because "everybody" could see what needed to be done except for the once-great leader.  

The same kind of thing nearly always happens to great leaders, sooner or later - unless they die or are compelled to retire (e.g. by illness or external removal) before this phase is reached. 

(Leaders of all kinds, including the greatest, very seldom or never seem to recognize their own decline and resign at the optimal time - probably because a personality inclined to such doubts would prevent good leadership in the first place.)  

And the removal of the once-great leader who has "lost it" is usually delayed to a damaging extent - exactly because of their prior greatness, and awareness that any replacement will be lesser in stature. 

This is presumably one reason why the "reign" of a great leader is so often followed by a slump of decline. 

This happens, not just from the lesser quality of men who follow; but also from the damage done to those-led, during this inept and misguided "terminal phase" of leadership.    

Why then do great leaders "always" decline? 

Well, there are of course many specific negative reasons - of which the change in personal character and ability inevitable with advancing age is surely one. 

(This is, after all, fundamentally and unavoidably an entropic world; and eternal life and creation can be found only post-mortally, in Heaven.) 

But I suspect that these are all secondary to the fact that any ability is a positive achievement, and the highest levels of human attainment always require multiple positive attributes to be present at the highest level and in a mutually-sustaining balance. 

Such high level of abilities and their harmonious alignment is extremely rare, and of course vulnerable. 

In other words, we ought to be grateful and delighted when great leadership emerges; but we ought not to be surprised that it cannot be sustained for long periods - and we should expect that sooner or later there will be a significant decline.  

Another example is creative writing, for example novels or dramas; in which there are many instances of an author who produce just one great work, and whose subsequent works never match their earlier pinnacle.

(What is rare, is an great author who produces many great works. So that, among English language dramatists of the past 500 years there is only Shakespeare and Shaw who reside solidly in this category.)  

The proper attitude is to be pleased that everything aligned optimally for long enough that the one great work was produced - but not to feel it necessary to "explain" why further great works did not emerge. 

Great artists are not factory assembly lines, which - once optimised - can churn out more-of-the-same quality of item, unless broken; and the same applies to great leaders.

Instead; with great leaders we may observe that for a while, there is a remarkable confluence of abilities with opportunities - and great leadership eventuates while such conditions prevail. 

But it is the confluence, the synergistic alignment, of all necessities at once which is remarkable - not the fact that such supreme coincidence of qualities cannot last forever; nor indeed (usually) for very long.