In the 70s it was all we had and I don't recall that anyone had any difficulty learning to it. When we got our first ten speeds, we just hopped on and started riding. Didn't seem awkward, it was just what you did.
I didn't ride at all for 30+ years and when I started riding again I thought modern shifting felt odd. So I got a vintage bike and it instantly felt perfectly natural.
I'm sure it does feel odd when you're used to something else, but I don't think it will take very many rides to get used to it.