In any tournament, there are different players. Some fight for victory, some objectively do not fight but think otherwise themselves, and others (like me) make up a not-so-unimportant crowd — playing for enjoyment/fun/gaining experience/learning. If you remove this last group of players because they have little chance of winning, it won't end well for the tournaments on HeroesWorld as a whole.
The issue of withdrawals, it seems to me, arises either from simple disorganization or from the second group. That is, a player registers with the desire to fight for victory, and when that possibility disappears, they lose interest in continuing to play.
I don't know the best way to solve this, but personally, I really like the Swiss system overall as a format. With a normal tournament progression under this system, players mostly compete against opponents of more or less equal strength at all levels. The Olympic and Round-Robin systems, of course, also have their own pros and cons. And it's probably good to have some variety.