Yes, Reddit should modify posts that contain profane language. There is enough profanity on the internet already. If Reddit wants to be considered a credible source for news and information, they need to be more professional about it. They should set a policy that either rejects posts with profanity or they should edit them.
Reddit is a site where all of it's users are free to do what they want, write stories, make funny posts, or even write serious thing, and in all of these cases they have the wright to use profanities if they want to, banning them would be censoring the freedom of speech that reddit users have, and that would be just unacceptable.
Most of the people on Reddit are old enough to be able to see profane language without it doing them any harm. Modifying someone's posts and comments is violation of somebody's freedom of speech. There should be a certain age you have to be to sign up that also warns users of profane language.
It is not right for Reddit to modify posts that contain profane language. If Reddit wishes to simply ban such language and delete the posts that is acceptable, however, it should not modify the posts. Either they allow this language, or they don't, and either keeping the posts or banning them entirely is the better solution.
There are a variety of safety controls on the Internet today. A person doesn't have to look at anything they don't want to look at. If a person doesn't like that there is swearing on Reddit, they can go to other websites. It shouldn't surprise anyone that Reddit has profane language on it.