Public transit should be worth the (inexpensive) cost of a ticket. The money used to make fares free should instead be spent on improving the quality of service. Better service attracts more riders, bringing in more ticket revenue, enabling further service improvements, and so on. It produces a positive flywheel effect that benefits current passengers, new passengers, and even drivers!

Free transit is a downward cycle, where increased ridership just increases costs, creating a negative flywheel.

Counterexample: Public transport throughout Luxembourg (the entire country, not just the capital city) is free. As in beer. Clean, safe, and reliable, too.

The positive "flywheel effect" where transport connectivity enables network effects does exist, but it does not follow that free public transport leads to negative effects.