Having the UI code in ROM doesn't preclude (and might actually benefit) shipping a simple but versatile sort of debugging tool with the OS. But Apple decided against doing so, making the system more appliance-like overall.
DOS's DEBUG isn't just a debugger; it also functions as a disk editor, hex editor, memory inspector, and general "system inspecting/tinkering" tool.
DOS's DEBUG isn't just a debugger; it also functions as a disk editor, hex editor, memory inspector, and general "system inspecting/tinkering" tool.