It is not impossible, it is possible that the mentioned team must be eliminated in the group phase. And the dark horse team drove unexpectedly, in fact this is what I look forward to at every biggest football party. Where countries that are not too favorites can talk a lot here. This is something that might happen in the current edition of the World Cup.
Who can be the dark horses this time, and do miracle like what Croatia did last time? No one really expected Croatia to qualify for the quarter-finals and they did the unimaginable by almost winning the trophy (back in 2014 it was Costa Rica). This time who will be the surprise package? I have made a list of teams who are capable of major upsets. Here is my selection - Senegal, Serbia, South Korea, Cameroon, Ghana, Wales and Australia. I am sure that one or more of these teams will feature in the semi-finals.
Your selection is very interesting but i'm a bit worried about African teams because in the previous tournament they were totally failed and none of African teams who able went to round 16 at that time but i interested to see Serbia team because this team unbeatable for qualification match and finish at first position on group A above Portugal so i think they will talk more in this tournament especially they have good player such as Dušan Vlahović from Juventus or Luka Jović who currently playing for Real Madrid so in my prediction Serbia achievement for this world cup at least will went to quarterfinal