The point is, legends aren't guaranteed to succeed as coaches and in fact, there's plenty of evidence of legends failing as coaches. Indeed, there are also some legends who have actually succeeded or at least are still able to deliver good results, but with that reason, I don't think there is a full guarantee for legends to be able to succeed as well when in a coaching career.
In addition, the fact that van Nistelrooy is currently also serving as an assistant coach at Manchester United and thus, it would be very possible in the future van Nistelrooy will replace the head coach if in the middle of the season the head coach is fired and if for example there is still no replacement. If such a case or scenario occurs, then ofcourse the assistant coach will replace him and thus, van Nistelrooy has a very large chance to become the head coach of Manchester United later.
I don’t think it is a certain that legend do not succeed in their coaching career but rather it is due to how they all start their coaching career some of them jump into this managerial career thinking that their playing days experience is sufficient for them and they don’t go for coaching school again which is wrong. Being a good player doesn’t mean you will make a great coach, been a manager is hectic and needs you to work on your tactical skills and communication skills this are things that a player doesn’t necessarily needs. There are currently great coaches that were great players in their days people Pep guardiola, Carlo Ancelotti and Also Zidane to mention a few. This guys went from being players to study on been a great manager and it worked.
The appointment of Ruud Van Nistelrooy as assistant manager is a strategic move by Manchester United to get him acquainted with the job before maybe anything happens, He was on his way to been Burnley head coach when united offered him this contract by accepting it shows that they have promised him a bigger role