But my main point was to dump super old and limited architectures when these days we can economically use modern architectures we use everywhere else (x86/ARM/MIPS, whatever). If we wanted to, we could literally hoist designs from 20+ years ago and use newer production technologies to make them embeddable.
Using mainstream tech stacks is very empowering. Updated compilers everywhere, many programming languages and stacks with huge communities everywhere, modern debugging tools at low/no cost, fast debug cycles, etc.
But there's little interest in this because which self-respecting hardware maker would commoditize its own products? :-D
The problem is not the processor, it's the lack of MMU and/or special DMA or interrupt engines and special GPIO. No kernel is potentially ported to such custom infrastructure, and if you have a very tiny flash, few of the ones which could will fit. (Say, target L4, porting Fiasco or OKL4.)
You can use a small ARM with integrated flash, RAM and LCD controller. Does it need MMU for Linux? There is family of products for that. Or use another family for bare-metal systems.
You can see how the thing starts to complicate regarding architectures/platforms.