Game cancelled
Even when the winners are clearly decided, everybody else still has to have their rank/standing determined, and this cannot be determined without all games being completed.
It is partly the player's responsibility to calculate that the arena might close mid-way of their game. But you're right, chess.com should present players with a notification of some sorts before they agree to another game that might be cancelled.
And the reason why you can't continue to play the game out after the tourney ends is because it's an in-tournament game. It is created within the bounds of the arena, and it must respect those bounds for all technical purposes.
The tournament also can't be extended. But maybe that might be an option in future.