Bluetooth is entirely unrelated to other computers in cars. In fact, it's just the car's "radio" component that speaks bluetooth. You can easily install such a component in 70 year old cars. You'd only lack the ergonomic steering wheel controls, which are generally connected to the "radio" directly.

Other car functions use computers for very different reasons. The most successful is injection timing, which does significantly improve performance. It's why we have extremely efficient small diesel engines and reasonably powerful 1L petrol engines.