Most of the teams that qualified for the main stage in the last T20 World Cup have not played in the World Cup before so popularity plays a big role here. If these teams qualify for the main stage in every World Cup, the people of their country and their government will support them, thereby creating a possibility of financial gain here. And as the number of teams increases, the tournament will be longer, the competition will increase and these tournaments will be more enjoyable for cricket lovers. As you said that in future cricket will be included in Olympics many countries will get opportunity to play and I think new teams will emerge from here too who will do well in cricket in future.