It wouldn't be an unreasonable conjecture to suppose Leica incorporated a frame line lever for so long was to sell lenses to amateurs. An experienced photographer already understands focal lengths and can visualise them, like if he had a Nikon, but an amateur is open to the 'what if' doubt, and a little lever can not only be charged for in the final price of the camera, but also bring in revenue.
The very fact it is no longer necessary is because the internet fills the void, with 'show me your photos' and questions about lenses that weren't possible before.
V