>It's an ancient design

My ass.

You can trace all these designs back forever. It's more "inspired by" than actual incremental revision in most cases. There is just about nothing but some vague shapes that look similar and maybe some bolt lengths that are common between a 4L60 and the TH350 era stuff.

>Further, and this is the important part, because the lifecycle of everything ICE is much shorter now, measured in years as opposed to decades, they never will.

The average car on the road is lasting longer as the years go on. People said the same things when fuel injection came out.

>So ten years from now, when your circa 2020 10L60 dies, there won't be a transmission shop in every town that's equipped and stocked to deal with it cheaply. The cost will be greater than the value of the vehicle.

I'll take that bet. Modern transmissions are stupidly easy to rebuild from a skills point of view because they replace all sorts of mechanical adjustment with "hurr hurr we just PWM the solenoid to make it go BRRT and if the BRRT is too rough the computer algorithm will turn it down". Yeah there's more components, but those are easi.