The films have overlap happening in the gap. The black bars just hide the fact that they don't line up, due to how lenses curve the edges of the image onto film. They used straight up 4:3 aspect ratio to reduce the curve but it's still there especially in the corners.
The reason I think the original would look like crap on the new projectors is that you don't need 9 projectors anymore to do 360. Heck consumers are MAKING 360 videos with just one camera and someone is making a single 360 projector. So the problem is that the original film is still just 9 separate films. That's a prime number. You cannot cut that down to fewer projectors without effectively adding more seams and thus smudging the image quality. I don't know what the industry uses for 360 anymore, but I think it's 5 cameras.
My point is we can only HOPE Disney is making new films that would blow us away like the originals did back in the 1980s!