Mr. Rogers having an open discussion with a post-WW1 / pre-WW2 Hitler would be fascinating and probably not useless.
The problem starts when Hitler is having discussions with only people who listen to Hitler. Without an interlocutor to say "wait a minute, that's crazy talk and here's why", it's easy to fall into an echo chamber of group-think.
Hitler was a manipulator. He would never come into a discussion with a rational, balanced perspective, or be open to feedback. He wouldn’t even really be speaking with Mr. Rodgers, he’d be using him to talk to disaffected, vulnerable people he can use to further his aims.
I agree that it’s interesting to think about a conversation between the two, but only if Hitler approached the conversation in the same way Mr. Rodgers would and unfortunately from history there’s no indication that would ever happen.
It’s banned in Germany. Not saying it solves the problem, but there are many different shades of free speech and every country I know of has at least some limitations.
People having open discussions with people I don't like and disagree with is "giving a platform" and must be punished.