When I got my first derailleur bike about 50 years ago, I was puzzled why the shifters weren't made to index just like the Sturmley-Archer trigger shifters I was used to. I learned that the reason was unreliability because of cable stretch and the obvious misalignment that happens as the front and rear chain line changes. Back in the eighties Suntour came out with shifters that mounted on top of the downtube which compensated for the front change by adjusting the rear cable and vice versa. I don't have a lot of experience with index shifters, except fine tuning the ones on my son's bike. It's certainly more finicky than adjusting the cables on friction shifters. I still don't know if index shifters incorporate a means of compensating for the change in chain line. It seems to me that index shifting was introduced to dumb down shifting so as to accomodate the masses flocking to the SUVs of cycling, mountain bikes.