Found this on ****** and it seems relevant to the conversation, especially the last paragraph:

”With friction shifters, the less dead space in between gears, the better. With a bad derailleur that gives a large pulley gap, along with straight-cut cogs and flat-plated chains, there will be very slow, sloppy gear changes that involve lots of noise and over-shooting. It's actually very difficult to change gears in these situations.

By contrast, say you took those exact same friction shifters and hooked up a modern derailleur (even Claris, but up to Dura-Ace) matched with a modern chain and cogs - even 5 or 6 speeds. Every touch of the lever would result in an instant, exact change from cog to cog. It's astonishingly improved.

At the shop I worked at this summer, I always told people the most cost-effective upgrade they can do on a vintage bike is swap the freewheel and chain to modern equivalents. The shifting difference is remarkable.”