There's the suggestion that great ex players, because it came so easily to them, do not have an understanding or the empathy to work successfully with 'lesser' players.
Coaching goes far beyond having played at an elite level, whereas those who have worked hard at their game throughout their careers, or have never reached the heights of Botham, etc have the experiences and knowledge of how to improve. There are examples of this across all sports. You can argue that it helps, that elite players will respect elite performers, but I would suggest that can work both ways. Respect comes from proving you can do your job, whether that's as a data analyst, physio or coach. Take Arsene Wenger - never played top flight football and when he went to Arsenal, got rid of many top players, such as Paul Merson, who thought he was ridiculous. You can speculate all day about his overall success but what he did at that club in the following years was amazing. brought in new methods that are replicated around the world and earned the respect of great footballers.
There's also the hunger - do these great players have it in them to knuckle down as it were and work tirelessly, arguably more so than when they were players, when it was 'easy' for them? Add to that tarnishing their reputation - why have unsuccessful management or coaching on your cv and for ever onwards 'great player, terrible coach?'
I'd take a successfully elite player becoming a coach - but I'd also want him to be 'current' i.e. not living off techniques of their playing time and who recognises the game has moved on...