Sure, you can replace brake pads yourself. But few people want to do it; let alone are capable. Saying "significant sources of income are therefore shrunk" significantly overstates just how many people would do their maintenance work on that level.

I know people who have paid people to replace the bulbs on a car. My wife rarely even bothers to refill the washer and I can count on 1 hand the number of times she's checker her oil in the nearly 20 years of owning a car.

Most people either don't care how cars work, don't have the technical capabilities to fix their own car or simply don't want to.

>I know people who have paid people to replace the bulbs on a car.

To be fair most newer cars need a fair amount of front bodywork pulled to change a bulb and most crossovers require pulling tail light housings which can be kind of intimidating for someone who's never done anything more than changed windshield wipers.

Engine and trans stuff isn't a significant source of income for them though. If anything ICE specific parts are a break even waste of a bay because for every person dumb enough to have a dealer do a timing belt out of warranty there's another person getting some leaking gasket replaced under warranty. Dealerships probably stand to make more money on EVs. Independent mechanics will probably lose out some though.