Re: "Because content is not taken down..
That's not actually true now, is it? Imagine a right wing muppet. You know the sort. Objects to immigration, believes global warming is a hoax, hero worships Donald Trump and Nigel Farage, uses terms like Feminazi and imagines himself terribly witty. Have you pictured the oaf? Excellent.
Well, said oaf is browsing the internet, visiting all the sort of websites that oafs like him like to visit - dubious subreddits, DonaldJTrump.com, Conservapedia, the Daily Mail, Fox News - and confirmation bias will ensure that he laps up all the bullshit and propaganda. Given that saner minds are unlikely to be browsing these sources, it's highly unlikely that his oafish peabrain will be troubled by an opposing opinion. Even if they did, and supposing they bothered to comment, the old perception filters would snap down to prevent his meagre intellectual faculties from being troubled by something so disturbing as a thought.
Of course, this applies equally to all the other oafs that you might find on the web. Would be terrorists, for example, and so forth.
So no. Deradicalisation is far harder, more involved, and costly that radicalisation in the first place. How to deal with it? Unfortunately, I suspect that censorship might have a place (in addition to costly deradicalisation schemes). Because, whilst freedom of speech is a wonderful thing, some thoughts (evidence based scientific research for example) really do have more value than others (racism, sexism etc)