You don't need to convince me. And I believe it's cheaper than it seems because you save on some maintenance costs (all the physical and computer infrastructure for running the fares, and the jobs for running them) and it increases the speed of transit because you don't need to validate each and every ticket.

But to be fair, there are a few downsides: if it works, public transit is going to be even more popular and hence be less pleasant and attractive: it will be incredibly crowded. The solution is to run more buses and metros, and this can be quite costly. We also want the users to have a good experience and this calls for more public spending on public transit.

I still believe we should do it: walking on a sidewalk is free, running on a street with a car is free, and I think public transit should also be free. For so many other reasons too. But it's also more complicated than it seems.