The concern is not that matches will be formally "canceled"; it's that the relative unavailability of an MTO, or the penalty associated with an MTO, will determine the outcome of the match and/or possibly help end it prematurely.
For example, what if the game you "give up" is at 5-6 in the third set, with sets at 1-all? Your choice at that point is to take an MTO, and lose the game and the set (and probably the match), or to play when you're hurt or feeling sick, and thus probably lose the game and the set (and probably the match). Either way, you're probably screwed. The system is sending a pretty strong "you might as well just retire" message in this scenario.
Also, I don't think it's true that the equivalents of MTOs always come with formal negative consequences in other sports. In every major American team sport, if a player is seriously injured on the field, the game simply stops for as long as it takes to attend to that player, whether that be three minutes or 15 minutes. The player's team is not penalized for the delay. And for less serious injuries or other problems, the player simply leaves the game and is replaced by someone else. In U.S. football, basketball, and ice hockey, the injured player can return later if fit. In baseball, the player is out for good, but the team still receives no penalty just because one of its players got hurt. But in tennis, since no replacement can be inserted, obviously the rules have to be more lenient with respect to delays while treating the player, since there's no other way to continue the match.