I think a lot of the sentiments I've seen here to the effect of "person in my social circle who doesn't even like theme parks has a negative opinion of Six Flags" doesn't realize that these people probably have an even less informed opinion of Six Flags than you do, because their opinion is likely just what they've heard from you. People who never go to parks aside from their local one only know about other parks from the things that their friends who do go to other parks say about them. Enthusiasts broadly seem to dislike Six Flags, so normal people who occasionally talk to enthusiasts end up with a negative opinion of Six Flags. It's really just a feedback loop.
Six Flags operates *a lot* of parks and many of those parks feel very different from each other. Some SIX parks are indeed dirty and poorly operated, but not all. Six Flags does many things well. Their Flash Pass system is far more elegant than Fast Lane, and it works well at every park. The same cannot be said of Fast Lane (ever tried using it at Cedar Point? Enjoy somehow still waiting an hour after paying hundreds of dollars).
The idea that *every* Cedar Fair park is always clean and always has great operations isn't any more true than the idea that Six Flags are all dirty and mismanaged. Six Flags Great America is one of my absolute favorite amusement parks anywhere, and they've won awards for their cleanliness. I'm also a big fan of Darien Lake. Both are SIX properties. Coincidentally, "dirty and poorly operated" was my experience when I visited Worlds of Fun, which is a Cedar Fair park. Six Flags also has two different parks with animal safari-type exhibits, so they have the experience and expertise to bring that back if they wanted to. Six Flags has IPs that are far more relevant to children than Snoopy (as much as I personally LOVE Snoopy, he's not culturally relevant anymore outside of the Christmas special).
Kings Island is going to be just fine, and I don't know that we have any reason to believe that Six Flags will even be added to its name (aside from perhaps "Kings Island - a Six Flags park". Both chains have things that they do well and I really do think their collective expertise can really help both chains. Remember, Six Flags and Cedar Fair didn't compete with each other outside of California. But they *do* compete with the likes of Disney, Universal, SeaWorld, and other destinations, and this merger gives them access to the resources to actually stand a chance at competing with those chains. If anything, this has created more *actual* competition even though there are fewer competitors in the game.