IMO you cannot make a focus group for art. I Am Legend had a choice; try to be art or try to be mindless entertainment. By making the focus group dictate the movie they decided to back down from their artistic vision in order to please mindless drones who didn't want to be challenged.

Sure, I would probably also be upset watching that twist for the first time, but it would have got me thinking. I'm more of an exception that I don't rate movies on the first go, but for most people that's what they'd do. The focus group only gets one chance to share their feelings and it's one of challenging their core beliefs resulting in being upset, a negative feeling. Hence a negative rating.

But all movies aren't supposed to be feel-good movies. The absolute best movies in history makes us think. Have you ever seen Wes Anderson or Robert De Niro or the like change their vision bases on focus group feedback? No, because they know the first impression and thus first feeling might not be a positive one and thus inadvertently translate to a negative rating.

There's a balance though, sometimes an auteur needs a reality check, take Coppola's Megalopolis for example. I haven't seen it yet, but from what I heard he kinda went off the rails on that one. We have yet to see if it makes us think down the road, but maybe he needed someone to say, "you know, this doesn't work." But not from hundreds of random movie enjoyers. From experienced film creators who know to look deeper. Writers, cinematographers, gaffers, sound mixers. People who know their craft and can say "this has been tried before and didn't work." Or if you're feeling adventurous: "we've never tried this before. Let's see if it'll stick."