You can't really say that's not how Tesla has developed their system, they have never deployed a system beyond level 2, which is minimally defined as a vehicle with lane keeping and adaptive cruise control.
With respect to the other systems, their go to market systems were level 3 systems, but they had no other choice because they had to demonstrate competence to the consumer as well as needed to generate some offset to their development costs. Tesla is a profitable company with a level 2 system and are capable of generating millions in revenue with a level 2 system, because the vehicles are owned by the operators.
I'm not saying they definitively won't deploy a level 3 system, I'm saying it is not necessary for them to do so because they can demonstrate a system reliable enough to be deployed as a level 4 system while it operates as a level 2 system. I also don't really see the purpose of a level 3 system for their business, because most of the utility can be derived by deploying an extremely competent level 2 system.
If they ever get to a level 4 system with current hardware it will be the most thoroughly proven autonomous system in history. It will be interesting to see how rapidly this current method improves, because it surpassed the old method in under 12 months of development while the other method took, I believe, 6 years.