I propose it is altruistic to be replaceable and therefore, those who strive to be altruistic should strive to be replaceable.

Non sequitur, there may be other considerations that outweigh the altruistic gain from being replacable. Such as acquiring a unique skillset - even if gaining that unique skillset may by definition entail making yourself much harder to replace.

I.e. it may be more altruistic to become less replaceable, even if replaceability itself is altruistic.