>>1789749You mean an indexing adjuster knob on the shifter itself? Those existed briefly at the end of the downtube era. In practice they're pointless though, you want friction shifting for the front even with indexing for the rear, and essentially all rear derailers have adjusters on that end of the cable.
To OP's question, aside from racing considerations, having shifters and brake levers together is more user friendly and most folks are better served by having a simpler tool that doesn't require as much skill and attention to operate. Personally I'm a downtube shifter appreciator, so much so that I built a bike with a modern 2x11 drivetrain and indexed downtube shifters (barcons on braze on mounts), and I do love how riding it forces me to be mindful of my shifting and pedaling and timing - but most people just want to ride their bikes and don't take pleasure in thinking about those things and fine tuning their shift technique.
