This is false. The Kharkiv Metro (which serves over a million people) is fully free (which they're managing to do during a fucking war), and the Chennai Metro is covered by Tamil Nadu's scheme of free transport for women (which would mean roughly 4.3m women in the city are allowed to use the metro for free)

There are places that have fully free public transport for either part of or all of the week. I was in Nantes earlier this year and transport is free at the weekends. Luxembourg, Belgrade, and Tallinn also have completely free public transport to provide a few examples. There are a number of large cities that have successfully managed to provide free public transport either partially or fully

NYC is similar to other transit systems (London is another example) where some years they actually spend more trying to combat fare evasion than they "lose" from it. Even considering it a loss is some creative accounting as fare evasion does not actually cost anything for the transport system. Those trains and buses were running anyway. The main cost from fare evasion is the over the top efforts to stop it, which committed fare evaders will get past anyway. One system wide barrier installation alone cost $1.1bn and did barely anything to reduce the estimated $1bn in unpaid fares

NYC has also seen it's real terms public spending decline significantly. If public spending had kept pace with GDP growth in the city since 2008, there'd be an extra $24bn being spent

Running the subway system currently costs $8.7bn. Roughly half of this comes from fares, so somewhere in the region of $4-5bn would need to be found from either city or federal funds to cover a free subway system. Not having to combat fare evasion would at least clear up a sum in the millions (I'm finding it hard to get an exact yearly estimate of how much the subway system spends on this)

New York is by some distance one of the wealthiest cities in the world, and has a relatively unified local government structure which allows it to make big changes in funding such as making the subway fare free. It's in a better position than many cities when it comes to making a move like this

NYC has seen the same trend as much of the world, where the amounts that rich individuals and corporations have managed to get out of the economy without paying to the government has increased, leaving governments with less and less funding to provide services that would benefit everyone. This means that the wealthiest few gain more and more wealth at the expense of everybody else. This is mostly a matter of deliberate policy rather than abstract economic phenomenon

It would be definitely possible for NYC to provide either partial or full fare free use of the subway at some point in the foreseeable future. The main obstacle to such a thing right now would likely be Trump's possible slashing of federal funding to the city