When I was 19, I got my first Leica, a IIIa with a 50/3,5. I couldn't afford another lens. For much of my university career, therefore, I had, yes, a Leica and one lens, though it was later supplemented by a 90/4. I shot a mixture of FP3 (I got 800 feet of outdated film free) and slides.
I also owned a Pentax with a standard lens and a zoom. I used them very little (too inconvenient to carry, compared with the IIIa) but I did use them occasionally: I wasn't being doctrinare about it. For that matter, I also used a Minolta 16 sometimes.
Which is why I think the 'one Leica, one lens' idea is bloody awful advice. If it comes naturally to take most of your pictures that way, as it did to me, then do it. Otherwise, forcing yourself to live someone else's fantasy is about as feeble an idea as I have ever heard in my entire life.
Whatever happens, don't be doctrinaire about it, and don't agonize about it. Worrying about something else -- anything else, especially something you don't have to worry about -- is a short cut to bad pictures.
Today, I have several Leicas and several lenses. Sure, I could live with one body and a 35mm lens. For decades, the vast majority of my pictures were taken with one body and one lens (though I used other bodies and lenses when I saw fit). But going to one body and one lens wouldn't teach me a damn' thing now, because I've already had access to other kit.
Cheers,
R.