You do realize that none of the best managers coach national teams, right? It is far less prestige and money than a top club job. Martinez failed as a club coach, so he had to go coach a national team to rehab his image. If he was even half confident in himself he would have ridden their WC run (meeting expectations, not surpassing them) and taken a high level club job.
But he didn't, because he's a coward, who knows he can only win if he has vastly superior players. Belgium will do well against the minnows of their group, but will undoubtedly lose to a France or Germany in the semis of the Euro. Then he'll take a job at a big club in a smaller league. Because that's all he can do. Same as Silva.