Since almost all shifting is done electronically on modern bikes, the shifting can be done anywhere on the bike or you can even have wireless buttons/sensors on your cycling jersey or on the pedals to shift up and down. It doesn’t have to be close to the brake levers. I am sure someone has thought of that. It’ll be interesting to see some hacks or something on the internet.

I'm about to try using the Archer D1X to electronically shift, using their provided buttons. That said, I'll probably mount them near the brake levers anyway as that is where my hands are most of the time (yes, I did consider downtube mounting ).

I'm waiting for neural implants, so I can shift the bike just by thinking about it.

One of the biggest advantages to all this is that people with arm and hand disabilities are open to a whole new world of options and I'll bet it would be easy to covert the buttons for biting with the riders teeth to shift and maybe develope electronic braking for riders lacking total arm use. And yes. there are special trikes that have foot controlled steering.

If you rode a single speed or fixed gear that wouldn't matter.

In terms of this question at one point I wanted to build a vintage style Rando bike with downtube shifting but instead of mechanical stuff I wanted to do Di2. It was a silly dream but would have been neat to accomplish. Maybe one day.

I have very weak thumbs as result of repeated work injuries, thus found it difficult to activate the thumb shifters on my mt. bikes. I then converted one to a Di2 system, the other to a SRAM AXS system. They work great. I cannot however, easily activate the thumb buttons on the top of the Di2 road shifters, the buttons are too stiff.

Maybe someone can invent an Autobike

FWIW, Shimano makes auxiliary shifter pods that can be installed in assorted spots on a h-bar. Designed somewhat for aero bars, I have seen the pods on the tops for climbing, on the end of the aero bars as well as near the brake levers on cow horns. Useful additions.