In the case, thebasic principal is "don't have two pieces of automated contro, code for the same thing which may conflict running on the same box unless you designed the code from he outset for both to be on the same box".
The reason is that the two may conflict with each other causing spurious control signals. Since the planned end-state for an EAP car is EAP/TACC software and not basic cruise control, it would, to make sense to do all,of the testing and code necessary to,have TACC software run alongside basic cruise control. To me, the basic cruise control for non-TACC cars gets overwritten by the TACC software when installed scenario makes sense. Not saying it couldn't be done the other way, but the way I'm describing keeps the control code as simple as possible and reduces need for additional testing. It would seem to me that is how Tesla would have done it given that they appear to,take the approach of putting everything in one big piece of code (I.e.m auto,headlights and windshield wipers in AP2 cars).