Having a MMU, even one without support for virtual memory, would have increased the number of transistors in the CPU and,

For graphics and sounds abstracting that without adding many more transistors would have enormously slowed down graphics and sound generation (for example, even on the more powerful PC, which had a BIOS to abstract away writing text to the screen, programs had to bypass that and write directly to screen memory to get decent performance)

So, short-term, it only had disadvantages.