> then ... in some places (maybe cars that people care about working on) it became cleaner again

Those cleaner looking engine bays are usually worse to work on. Not better.

When you open up the hood and immediately see lines everywhere, that also means they’re within reach. This is great.

The engine bays that look nice and clean for the showroom still have those same lines. They’re just buried in there. If you need to work on them you’re going to be reaching underneath things, climbing under the car, or even removing other parts to access something simple.

Also, it’s not all about emissions. A lot of those lines are for modern comforts like cruise control and improvements like features that make cold starts easier or make the engine behave better at extreme temperatures. Some of those have been superseded by electronically controlled versions which is why some of those lines are disappearing on modern cars, but the overall complexity has increased further.