You do have machine code monitors available for a lot of these computers, though. E.g. on C64, if you want that workflow you'd almost certainly have a freezer cartridge that almost universally had a machine code monitor built in.
You'll also have full macro assemblers with built in editors that'll load nicely out of the way that only becomes an issue if you're seriously RAM constrained.
You'll also have full macro assemblers with built in editors that'll load nicely out of the way that only becomes an issue if you're seriously RAM constrained.