〜ばそれまでだ・〜たらそれまでだ
if …, that's the end of it
Verb ば / たら + それまでだ
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ばそれまでだ means that if a certain thing happens, everything else becomes pointless: いくら練習しても、本番で失敗すればそれまでだ, "however much you practise, if you fail on the day, that's the end of it".
The first half is often a single decisive event: a failure, an accident, spending the money, losing your life. The sentence stresses how fragile all the effort or value was. The first part often has いくら〜ても or どんなに.
The fixed phrase 言ってしまえばそれまでだが means "that's all there is to it, but…", used when a simple explanation doesn't feel satisfying: 運が悪かったと言ってしまえばそれまでだが.
それまで means "up to that point", so it's literally "it only goes up to there".
Examples
いくら練習しても、本番で失敗すればそれまでだ。
However much you practise, if you fail on the day, that's the end of it.
どんなに高い車も、事故を起こせばそれまでだ。
However expensive a car is, one accident and it's all over.
いくら貯金しても、使ってしまえばそれまでだ。
However much you save, once you spend it, it's gone.
どんなに成功しても、命をなくしたらそれまでだ。
However successful you are, if you lose your life, that's the end.
運が悪かったと言ってしまえばそれまでだが、準備も足りなかった。
You could just say it was bad luck, but we didn't prepare enough either.