They decided to change the format to slightly different from what Infantino proposed because if there were only three teams in a group, there are more chance for two teams to make an silent agreement to help each other to pass the group stage. The proposed format was that all groups consist of three teams and that two of them advance to the next round.
Personally I don't like the fact that there will be more teams in the next World Cup. What's the point to bring more teams to the World Cup? It will result in more poor games that will be unwatchable just like it was with games when Qatar played in this World Cup. It's nonsense to me...
Agreed with the first part of your post, but not with the second.
Groups of 3 teams make no sense. Even if there is no secret pact between two of the teams, such small groups will take away the excitement of teams gradually progressing to the knockout phase. Just two matches in the group phase? There is no logic here. They can make the teams play against the other two teams twice, but then it will make the tournament extremely lengthy.