Al Franken

Richard Stallman

Sinead O'Connor and the Catholic church

Not sure if this counts, but there was a militant trade union leader in the 80s called Arthur Scargill, who made many public speeches predicting what was going to happen to the British steel, coal and manufacturing industry.

He was widely derided, demonised and lampooned as a 'looney' by the Murdoch press.

Needless to say, pretty much everything he predicted came true.

I'm not sure if this is exactly what you're asking for but Nick Reiner comes to mind. He wrote a screenplay to spread awareness of the problems he saw with the rehab system. There were probably addicts who found comfort in the film and found it relatable. But now he has lost public sympathy because he murdered his parents, forever tainting the film and its intended messages.

https://en.wikipedia.org/wiki/L._L._Zamenhof

English is not my native language. I'm glad I learned it, because I just have an interest in linguistics and it's very close to my native language, but I can clearly see how "imperialistic" this is, how my parents and friends struggle with that language, and how there could be something much easier, that would allow them to experience cultural exchanges without having to learn the "dominant" language.

Mi lernis Esperanton pli ol 20 jaroj. Ĝin estus mirinda, sed malĝoja, mi malrememoras multa, kay uzas Anglo kaj Germano pli.

Me.