Remember that in the 70's and earlier there were no mountain bikes. Therre were English 3-speeds, their were single-speed cruisers, and their were "10-speeds." And that was generally it.
People who wanted more than three gears bought 10-speeds, and all 10-speeds had drop bars.
But then, as now, not everybody was comfortable with drop bars. So they rode with their hands on the tops of the bars almost all the time, and it was for them that safety levers were developed.
Today, the market for multi-gear bikes includes plenty of options for people who prefer upright positioning, so the need for cheater levers isn't there.
A modern road bike should really be sized so that the rider is not too stretched out when his/her hands are on the hoods of the "brifters," so that moving to the tops provides something of a rest position while moving to the drops is a more aero position. The "on the hoods" position should be the most comfortable for extended periods, IMO, and from that position on a modern bike one can, of course, both shift and brake without repositioning one's hands.
RichC