Could Tyrell have been telling the truth and it was a breakthrough that allowed extended lifespans which we see in 2049?
If he was lying, is the idea possibly that as replicants aged they became less reliable? And if so, was the real breakthrough making much more reliable replicants (even if such replicants were also not complete obedient)?
Tyrell wasn't lying.
It would appear that early model Replicants had a perfectly normal lifespan and that the newer Nexus-6 model Replicants were intentionally given a shorter lifespan in order to prevent them from developing their own thoughts and opinions and then going rogue.
There's also the suggestion that their enhanced strength and durability may also be connected to their shorter lifespan. Assuming they live lives that are twenty times faster than an average human, that would (pseudo-scientifically) explain their rapid healing, exceptional strength, etc.
Tyrell: ...all of this is academic. You were made as well as we could make you.
Roy: But not to last.
Tyrell: The light that burns twice as bright burns half as long. And you have burned so very very brightly, Roy. Look at you. You're the prodigal son. You're quite a prize!
It seems as if the breakthrough that allowed them to remove Tyrell's restrictions on lifespans for Nexus-7 and Nexus-8 models was the improvement in memory insertion (pioneered in Rachael) which meant that new model Replicants were more loyal and less likely to go rogue. They don't appear to be less strong, so there's possibly some element of improvement made in their physical structure also.
Either way, there's no good reason to assume that Tyrell was lying. It makes sense that an existing Replicant can't be given a longer lifespan, they can only be bred with one from scratch.
Note that the above is explicitly confirmed in the film's official novelisation. Tyrell gave them a shortened lifespan in order to control them, then later regretted his decision and worked (unsuccessfully) to try to remove this restriction from his existing models.
"Tyrell [said Bryant] was worried that the Nexus Six might be too close to human. Given time, a Nexus Six might develop emotions —love, hate, anger, pity, garbage like that. So Tyrell added a gimmick. A real cute gimmick. After four years every Nexus Six goes dead.” Bryant pushed the button. The screen went dark, and the room lights came on. "But they still can raise a lot of hell between now and their termination date. Get hopping, Deckard."
...
"And what can your Maker do for you?” asked Tyrell. "Life. I want more life. You didn’t give me enough."
"I tried, believe me,” said Tyrell. He patted Batty’s shoulder soothingly, as if he were calming an upset child. "I realized what a mistake I made by limiting your greatness. I did everything in my power to change your coding. But by the second day of your life, there was no altering the day of your death.”
