Could you materially affect a half-century old internal combustion engine? Sure. Can you do so after decades of miniaturization/optimization, to make it as efficient as they are today?
Mobiles are similar, they are filled to the brim with various electronics, connected together into a huge mash. why would you even expect to fix that?
This is a bad analogy because the hardware in engines of today is actually not that different or hard to work on fundamentally, but manufacturers do intentionally lock down software to make diagnostics very tricky. They became more efficient and complex, but people still hack on even the most modern engines, usually by tossing the OEM software.
Aftermarket ECUs (even the open source ones like rusEFI and speeduino) show that you can actually do the stuff required to make modern engines go vroom, but manufacturers have no desire to make that process easy out of the box.
They're much more complicated today. Much more. My Dad rebuilt the engine of our old Morris Minor, but even in the 1990s he would say he wouldn't have a hope of doing the same thing in a modern car.
OK so why don't these companies let the users try? If you are right, they won't be able to do it anyway, so no harm done. Why do companies use every technical and legal trick in the book to prevent people from even trying? It's obviously about what makes the company more money.
Mobiles are similar, they are filled to the brim with various electronics, connected together into a huge mash. why would you even expect to fix that?