Referring to the question here, the first and second paragraph.
On Thursday at 2pm GMT, the football matches for group H of the World Cup 2018 will begin. By 4 pm GMT, the fate of that group will be decided.
Let us assume that Japan comes first and Colombia comes second (not an unlikely outcome, actually), both hence qualifying for the second round. Now, at 6pm GMT, the matches of group G begin. Belgium and England are already qualified for the next round, but they play against each other in order to decide who goes first, and thus, to decide whom are they to play in the next round.
The problem with this arrangement, as mentioned in the question, is that England and Belgium can be unsportsmanlike in order to receive a more favorable opponent in the next round.
I know that the teams involved in unsportsmanlike behavior can be investigated and punished, but still, why FIFA not just decided to start all four matches simultaneously? This would reduce the possibility of deliberate losing.
Another alternative solution is that FIFA can consider to let all the qualifying teams to draw the lots again after all group matches complete, so that no one can "choose the opponent", why this is not done?