“it’s unlikely that Tyrell would have given Deckard an extended life but not her”
Tyrell did give Rachael an extended lifespan. She may have died of unnatural causes though, especially since she was probably being hunted down.
Only in the theatrical release. (The one with Harrison Ford droning on in voice overs) The studio insisted on a happy ending. In the extended/director's cut versions, she is not described as having a long lifespan. They only thing they say is "It's too bad she won't live... but then again, who does?"
Deckard & Rachel being replicants without expiration dates:
Rachel is of course a replicant, and there is a question of whether or not she had an expiration date - negative in the theatrical release, possibly yes in the director’s cut.
Deckard being a replicant was never explored in the theatrical release; it was assumed he was human. However, in the director’s cut, there are many clues that he is a replicant.
The movie explores what makes us human, and if humans could be duplicated. Implanted memory is where the rubber meets the road.
It’s a philosophical question at least as old as Locke - have you as a human being existed for a certain number of years, or were you fabricated and all those years are actually memory data somehow downloaded into you?
The big question in the film is why would Tyrell Corp create replicants having memories and no expiration dates?
Simple. Immortality. If you can fabricate a body and download memories into it, then the human experience can hypothetically be extended into infinity.
Then it becomes a matter of control, of power. He/She who controls access to immortality controls sentient life.
Rachel is an illustration of the first principle. Deckard (whom you notice comes into the world fully formed, with no memories depicted on screen) illustrates the second by being an enforcer replicant.