I think we should stem from the Motto of the Tyrell Corporation "More human than human".
Replicants are used off-world as slave labor.
And, from what I gathered from the movie, any design by the Tyrell Corp. before the Nexus 6 was simply; basic "Do as I say, android" Replicants.
The Replicants have a 4 year lifespan because, and this is looking back at our own world history, people tend to revolt.
There are stages to slavery;
1. Those that accept and continue to live a life of servitude (because perhaps they know their lives would be much harder without being taken care of by their masters).
2. Those that are oppressed because they are frightened into submission, but once acclimated become the leaders of men towards revolting. For they are the ones who see there is more to life than servitude.
I see their 4 year lifespan as being...
Birth: "A strange new world, it's amazing."
Take any child, from birth, and teach it to get your slippers, bring the news paper, a beer, etc... As long as you instil Fear or an unconditioning of Love (yes love can and will be use against you. It's called manipulations) they will forever be your lapdog.
Savery: Being used for which they were designed.
Show them a world outside of what they have grown to know and they become defiant.
Revolting: Their wakeup period as to the real life around them and wanting to change the way they live.
So rather then have any replicants form a giant revolution and start wars of their own, and towards Replicants being recognized as a species that should have rights (I believe DADOES touches on this base a bit), Tyrell simply implants a fail-safe.
Plus: Every good company knows about Supply and Demand.
The Tyrell Motto implies that they are designed to specifications of the operator for their menial tasks.
Need soldiers, you got super soldiers.
want to run an off world brothel, you got sluts that give you no lip.
It really comes down to slavery/human trafficking, once that person has become useless they are no better than garbage.
So in essence, "More Human Than Human" is an overstatement.
It wasn't until the Nexus 6 was introduced with memories to help them "Cope with their existence".
If I remember correctly, this is when the Blade Runner unit was formed.
Because a group of replicants revolted and killed hundreds on a starship heading towards a new planet for terraforming.
Of course DADOES doesn't reference the term "Blade Runners", but since the movie is so loosely based off the book it's as if Blade Runner is in a parallel universe that mimics DADOES (think of the show Sliders).