This pattern conveys that the action or state described is the only thing to do, and there is no alternative. It often carries a nuance of 'that's all,' 'nothing more,' or 'might as well.' It is used when the speaker acknowledges a situation and decides on a simple, final course of action, sometimes with a sense of resignation ('if that fails, that's it') or determination ('I'll just do this').

The two forms are essentially interchangeable, but までのことだ is slightly more emphatic or explanatory, often adding a tone of 'there's nothing else to it.' The pattern is typically used in spoken or written discourse to present a conclusion or a fallback plan. It is not used to describe a habitual action but rather a specific decision in a given situation.

Note that the pattern is somewhat formal and literary; in casual conversation, you might hear it in set phrases like それまでだ (that's it, game over), but the full forms are more common in writing or formal speech.