Look at the video on the last page about Nagelsmann, great young manager with new ideas, came through youth setup, did his apprenticeship with youngsters, brought through youngsters into first team.... It's pretty much the same pattern with all the young managers that people want, came through youth setup, did surprisingly well.This ties in quite well with what @Shropshire Blue said about assessing a manager's past constraints and effects on a player.
The reason that these youth team coaches have been successful in Germany? It's probably got a lot to do with the fact that, as they've already developed youth team players in to first team players, they've already demonstrated success with coaching both those players and the club's style. They've shown that they can communicate well with the younger players and impart their ideas.
Our next manager has to be able to work well with and coach the players we've got. Koeman's plans A and B revolved around ball playing target men, giving him the option to launch it forward when things weren't going well. It's a big reason that it didn't work for Koeman at Valencia, for example, where he was stuck with 5 foot 9 David Villa up front as Morientes was injured for most of his reign.
Most managers can't or don't alter their styles too much. So - given the difficulty of signing good quality players in January - our next manager has to be able to work with what we've already got. Our board probably needs to work out what our future strategy is going to be too though, we can't risk another situation where Walsh and the manager are seemingly targeting different types of players.