The problem is, the further you change the CPU, the more work every dev who has ever released a game would have to do to ensure compatibility on the new hardware with the X1.
Sorry but what you are saying is not entirely correct (subtle saying).
Lets go into details:
a) Instruction set: Zen and Jag just use the same instruction sets. You don't even need to recompile your source code for the Zen.
No issues with compatibility by changing the CPU!
b) Zen probably has a better IPC which means the CPU is faster in doing its calculations. Unless Devs have not put a single effort in synchronizing their threads in their existing games and totally forgot about that:
No issues with compatibility by changing the CPU!
c) Memory speed limitations due to GDDR5 instead of DDR3. This is independent from the CPU architecture they use, it is in fact the same situation for Jag and Zen.
No issues with compatibility by changing the CPU!
d) number of cores: MS said 8 cores. I tend to believe they always talked about physical cores. so no difference here.
No issues with compatibility by changing the CPU!
e) if they would use Zen SMT: This is the only part where you can run into issues as you can't be on premise like with a physical core. BUT, the increase in IPC over Jag is more than enough to make this an non-issue.
So please tell me where you see those compatibility issues you are talking about. I can't see them on a hw level unless somebody programmed his game in Assembler.
Edit:
Btw I am not talking about a game is optimized to a certain CPU I talk about that a game can run on Zen like it does on Jag without any issues! I also saw your stance on how to handle the different situation about ESRAM. Thats far more challenging than changing the CPU because of the difference in read/write behavior of the memory for the GPU.