1) A team reaching the playoffs is attractive for potential investors/sponsors, even if it's only the quarterfinals. For not-so-rich leagues, that's already good in itself.
2) Not every league has 14, 16 teams with money to play in the main tournament.
5) The 5th beating the 4th not being a surprise is enough to put down the argument that QFs are useless.
1) they are still reaching to the playoffs, but for 5th place. When they are eliminated with a clear score in playoffs it doesn't add much to their prestige. Instead of being eliminated in quarterfinals, they can call them 5th or 6th of season.
2) this point is irrelevant. what main tournament are you talking about? It is a playoff system but with less teams. There is almost no major league with 16 teams. Most of them are 12 and only a few are 14 anyway
3) the fact that it is possible doesn't mean it is probable. Most of the time still 4th is winning over 5th. If you read my previous posts, you will see thay my ideal case is 6 teams play off system with 3, 4, 5 and 6th playing elimination round.
The league is a long marathon, there should be a reward for consistency. With 8 teams playoffs, teams are very vulnerable for injury or COVID.