>... a 40-year-old 16-bit ISA designed to be source-compatible with a 50-year-old 8-bit ISA.
In fairness to the Intel of that era, they actually did a really good job with this. They gained basically zero warts from the 8080 assembler source compatibility. They mostly set out to make the best variable length 16 bit instruction set they could. They had significant competition at the time and they pretty much had to make the 8086 instruction set something very usable. The x86 instruction set horror mostly came after that.
In fairness to the Intel of that era, they actually did a really good job with this. They gained basically zero warts from the 8080 assembler source compatibility. They mostly set out to make the best variable length 16 bit instruction set they could. They had significant competition at the time and they pretty much had to make the 8086 instruction set something very usable. The x86 instruction set horror mostly came after that.