I have ridden using downtube shifters since the early 1970s. As others have said, there was a time when friction shifting on the downtube was the most common type on better bicycles. Bar end shifters were also in use, even by professionals. We all learned to use them because we didn't know anything else. There were subtle differences between brands in how much you had to "overshoot" the downshift (toward the larger cog), and then back off the lever. I'm looking at you, Campagnolo, and to a lesser extent SunTour. This had to be done by ear and by feel.
Fast forward to the mid-to-late 1980s when indexed shifting emerged, and that made a marginal improvement in how rapidly one could move across the rear cogset. Then the shaped teeth on the Hyperglide style freewheel changed everything. The chain jumped and settled onto the correct cog very quickly with less fiddling, even using friction shifters.
I have three friction shifting bikes now: two on the downtube and one with bar ends. They're fine for what they are, and are enjoyable to ride.
But after saying all of this, I prefer STIs for their ease of use. This is especially true in cold weather with cumbersome gloves or mittens. Plus, with added hats, and layers on the head, it's tougher hear the mis-adjustment of slight errors in friction shifting. With indexed shifting, you don't worry about that as much, if at all.