Against substitutions. During the team formation phase, a very conditional, but still a kind of balance is created. Any substitution can significantly disrupt it.
For example, in the last tournament, there was a team consisting of Toganash, Dima_Surmai, and Yu2nub. Not the strongest first tier, a good second tier, and a very strong third tier. The team easily passed the group stage. And then Toganash disappeared (and, by the way, he had the audacity to submit an application for a new tournament after that). A weak player from the first tier is replaced by an average one (at least) - Wowa. As a result, the team easily makes it to the final, where they lead 2:0 in the best-of-five series. Luckily, they managed to pull it off in the end, otherwise, I would have been very upset :)
So, I suggest doing this: if someone drops out, then a player from a lower tier can play in their place. For example, if the captain drops out, any of the remaining players can play in their place (decided among the remaining team members); if the "number two" drops out, then players 3 or 4 can play in their place. But if the fourth player drops out, then, unfortunately...