“maybe they couldn’t extend a replicant’s life after it was created”
That is the problem. This is why Roy suggests using retroviruses, since they can rewrite DNA of an already living organism.
“But if that was the case, what was so special about Rachel?”
She was designed without the shortened lifespan. Apparently Tyrell could only create new replicants without an artificial expiration date, but he could not “fix” the ones he had already created.
The Director’s cut took that out. It simply ends at the elevator scene, with more emphasis on the origami unicorn thus hinting more strongly that Deckard might be a replicant (because Gaff knew about his dream).
Ahhhh...that bit of dialogue always went over my head. Thanks!