User is definitely a term that long predates the modern SaaS world. And it’s an appropriate term in many cases because even today the customers of a computer hardware or software company are often not the same people using that hardware or software. I am the user of my work computer, but even as a software developer I am certainly not the “customer” of that purchase. My company has requirements as a customer that might be counter to my desires as a user. And likewise I have needs as a user that my company as a customer does not care about (except in so far as having those needs met allows me to do my job)