What do you want? Do you want a system to play with for your own learning? Or do you want to build a shipping product?

If for your own learning, do you want a full-powered environment? Or do you want a simple system that you can learn all of, even if it's more of a toy?

If it's a shipping product, do you care more about ease of development, or about total parts cost? (The difference is often quantity that you expect to ship - 10 cents in part costs matters if you expect to make 100 million of them.)

Yes 8 bit is absolutely a good choice. ARM comes in less than a dollar for the cortex m0 core (maybe others now?) but sometimes that is more than you need in both size and capabilities.

Also sometimes you may have existing expertise, tooling, or firmware already for a processor such as a PIC, AVR, or STM8, so why reinvent?

Some examples may be things like a custom serial to input/output expander, small motor controller, power controller, data logger, or other small and simple machines.

Depends on your cost/power/space budget. There's plenty of new 8-bit work being done today.