Ever heard of a loop that needed to keep more than 7 variables live? Register renaming helps with pipelining and out-of-order execution, but instructions in the program can only reference the architectural registers - go beyond that and you end up needing to spill some values to (architectural) memory.

There's a reason why AMD added r8-r15 to the architecture, and why intel is adding r16-r31..

That's irrelevant, the zero register would be taking a slot in the limited register addressing bits in instructions, not replace a physical register on the chip.

8086 doesn't have that.