If 20 teams get a chance to participate in the World Cup, the quality of the World Cup will further decrease. Because the performance of other teams is quite weak except for the first 8-10 teams of ranking. Teams like UAE, PNG, and Hong Kong will never win against the big teams and every match will be one-sided. World Cup matches will not be competitive. So viewers will lose interest in watching the World Cup. But maybe teams like UAE, Hong Kong, and Nepal will try to strengthen themselves to perform in the World Cup.
Your allegation has no basis. Some of the most competitive matches in the T20 world cup involved associate nations. And if you check the history, you can see that a majority of the lop-sided matches in the tournament were between two tier-1 test nations. The excuse of low quality is raised by lower ranking test nations, because they are afraid of getting beaten by associate sides. During the last world cup, Bangladesh was defeated by Scotland, and Namibia managed to defeat Ireland. If you want to maintain quality, then the tournament should comprise only of 4 teams - India, Australia, New Zealand and England.