At the World Cup, Belgium was very bad in the first game against Canada, but after that they raised their level of play and were much better, especially in the game against us when they were a much better team. Only luck and the miraculous Guardiol saved Croatia from a well-deserved defeat.
Some say that Belgium is an old team that is no longer good, but it is a very experienced and high-quality national team that should not be underestimated.
The German national team is definitely in a big decline since the 2014 World Cup and they are not favorites at the next EURO, even though they are the hosts.
The Croatian national team is always dangerous and unpredictable, and has a chance to make a big result again.
I don't think that they are old as a team. Their average age is 27 years. Even their best players are all under 30 years. I wouldn't say that they are old. I didn't watch all their games, but the games against Canada and Morocco were pretty bad. Maybe they just underestimated them. But if you look at their last game against Sweden, Lukaku scored all three goals. They depend on him a lot and if he is not at his best, or if he gets injured, it would be catastrophic for them.
I'm curious to see how Croatia will play once Modrić won't be playing for the national team. I don't think that Croatia depends on him as much as Belgium depends on Lukaku.