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Depending on what you're talking about, "by" can mean "before", "no later than", "on", or "since".
"Avant" and "arrivé" both imply that when 1950 dawned, they were already dead, so they're probably too precise.
"Passé 1950" would have left a little more uncertainty, but "by" is a simple word and probably deserves a less colourful translation.
"Dès" seems sufficiently vague to leave the reader the choice of what the exact moment was, in or before 1950, when the last whatever-it-was died.
I agree with you that by is difficult to translate and is very dependent upon the context. In many instances the only solution is to use a "periphrase"
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