This pattern expresses that if a particular condition is met, then all previous effort or the entire situation becomes meaningless—it's over. It's often used in a resigned or practical tone, pointing out a fatal flaw or a point of no return. The ば version sounds slightly more formal or literary, while たら is more common in everyday speech. Both are used in written and spoken Japanese, but たらそれまでだ is more natural in conversation.

Think of it as saying "if that happens, then that's the end of it." It's not about a positive outcome, but rather a negative, dead-end result. For example, if you study hard but fail the exam, all that studying was for nothing—that's せっかく勉強したのに、試験に落ちたらそれまでだ. The pattern highlights the fragility of success or the finality of a failure.

Use it when you want to emphasize that something is a decisive condition that nullifies everything else. It's a useful pattern for expressing risk, regret, or a realistic warning.