Paul, my take on this is that with the modern baits (tools), depth and speed can be independent of each other. I primarily fish swimbaits, so I'll draw an example from there.
I take an ROF5 hudd (6" or 8" or 68- different sizes, and actions), remove some lead from the belly to make it an ROF2-ish. I can (and do) fish it very slow subsurface, a couple feet down in the column. Real trouts seem to do that a lot.
I can also crawl it uphill along the bottom (after letting it sink), at the same speed.
If I want to fish fast on the bottom, I do so with an ROF16 hudd. I can also fish fast a couple feet down with a slower sinking bait.
Maybe, the tools back in the day were not versatile enough?
Also, I'm talking about casting, since that's pretty much all I do.