One embedded platform I worked on was developed in the late 80s, launched in 1990, and code I wrote in 1994 is still running. It's a 6809 processor with a little bit of RAM and 512KB of EEPROM attached. The entire code base is pure assembly language. That doesn't mean it doesn't have bugs, it just means no stdlibs to lean on.

The key isn't the longevity of the CPU, it's whatever is attached to it. Input and output stages fail, power supplies spike, things burn out, things wear out. So the processor can keep going strong...but what are you controlling?

But TDLR the answer is use the oldest sustainable tech you can find. Think like Gunpei Yokoi and build a Gameboy, not a Steam Deck.