

While I am broadly speaking in favour of free transit, one thing that should be noted in any conversation about it is that it often does not significantly increase ridership by users who were previously not using transit - the increase mainly comes from increased utilization by prior users, and sometimes you can get negative effects such as the newly free transit trips displacing walking.
Hence, other investments into the transit system may be prudent - for example, expanding the system, increasing headways and so on.
We should try to be brave enough to do all of the above, but sometimes you’re forced to choose one or the other.

My very limited experience says that it’s possible but risky and unpleasant. NYC is probably a bit different, as is always the case in the U.S.
Walking works quite well in Sweden however