A few things here, wd40 actually accelerates the wear on your bike's drivetrain. It's sticky and will attract grime which chews away at your chainring and cassette. It also gets all over your pants and stains your leg. Use some proper chain lube and your drivetrain and clothes will last longer.

If you ride your bike a decent amount you'll need to do morem maintenance. Per mile ridden, cheap Walmart bikes require more maintenance.

Complexity of the bike is also a huge factor. Compare the maintenance needed between a fixie with no brakes and a full suspension mtb with hydraulic disc brakes, tubeless tires 12 gears etc.

Adjusting brake cables and derailer limits needs to happen all the time, probably once per 500 miles. Over time chains stretch and then you need to replace the chain and the cassette.

Of course this entirely depends on how much you ride, where you ride, and what bike you ride.