I can understand why french posters are up in arms about some comments here when that is a usual thing occuring in french football.
I guess it very much depends on the country. RM and Barca did it with greath success, yet this thing never happens at a top 6 club in England. They always want the proven thing from the outside.
In Germany it is quite common to appoint the U23/U18 manager as an interim manager and give him the permanent job if he does well. But, this never happens at the biggest clubs, almost exclusively at mid table / lower table clubs. To have a rookie coming from outside the club to take over is very rare. The big clubs especially like to get managerial talent from smaller clubs whenever they can, also because getting foreign talent is very difficult if you are not Bayern Munich or if it‘s not a german speaking manager (swiss / austrian / dutch / former BL player like Kovac).
The biggest clubs like Bayern, Dortmund, Schalke never go for the youth manager solution, they want someone who has shown great performances at a smaller club first. Bayern fans cried when they signed Kovac because they thought he wasn‘t proven, despite winning Frankfurt‘s first ever title in 30 years, and that wasn‘t a fluke if you factor in that he got Frankfurt to the Cup final the year before as well.