Batty: Why not?
Tyrell: Because by the second day of incubation, any cells that have undergone reversion mutation give rise to revertant colonies, like rats leaving a sinking ship; then the ship... sinks.
Batty: What about EMS-3 recombination?
Tyrell: We've already tried it - ethyl, methane, sulfinate as an alkylating agent and potent mutagen; it created a virus so lethal the subject was dead before it even left the table.
Batty: Then a repressor protein, that would block the operating cells.
Tyrell: Wouldn't obstruct replication; but it does give rise to an error in replication, so that the newly formed DNA strand carries with it a mutation - and you've got a virus again... but this, all of this is academic. You were made as well as we could make you.
Batty: But not to last.
It was true that Tyrell couldn't fix Batty, and true that they made him as well as they could at the time he was made, but he may have left out that they had figured out how to make replicants better after Batty was made. If that was the case, the information wouldn't have saved him, so he probably wouldn't disclose it.
Notice what is missing in this statement: "A coding sequence cannot be revised once it's been established."... what about before it's been established? Such as if the design was refined before making a new model of replicant, such as Rachel and Deckard seem to be?