Soarin' was shot and is presented on IMAX 70MM film, at a framerate of 48fps, which is double that of a standard film--yes, even digital.
I know that a lot of people don't know what this means, but fortunately I have the expertise to elaborate. First, some background. Before the advent of HD and digital presentation, the standard format for shooting and presenting films (the ones you see at a regular movie theater) was 35MM film. That is half the physical size of IMAX film, and it is shot at half as many frames per second (24fps). That format, of standard 35MM film, is a superior format in image quality to HD video. That is a fact, not an opinion.
Full HD is defined as an image measuring 1920 pixels in width, and a maximum of 1080 pixels vertically. That looks impressive compared to the old SD standard of 640 pixels wide by 480 pixels tall, which is what your old "tube" television presented, but it is a major downgrade from the resolution of 35MM film. And the really sad thing is that movie studios have actually started presenting movies in theaters under the "digital" moniker at this lower resolution, having the gall to advertise it as a superior cinematic experience. They have really sold the public on that notion, so I can't blame you for thinking that "HD" and "digital" means "better than film." If you doubt this, just go sit in the very front row the next time you go to the movies, and then just look at a corner of the screen rather than taking in the whole picture. Look for the pixels. You'll realize that they are there, and they might even be as big as the extra large Coca-Cola you're holding.
Now, stepping back, the IMAX that Soarin' uses presents DOUBLE the resolution AND framerate of 35MM film, which is itself already superior to any digital presentation available anywhere. There really is no comparison.
I will admit one caveat: Film, including IMAX film, does require maintenance and cleaning. Even then, you will occasionally see some dirt. The type of IMAX projector they use for Soarin' happens to be the same kind that I have worked with, which uses an air compressor to blow such dirt out before it can be seen, but some will still slip through. BUT, would you rather the occasional fleck of dirt, or pixels all over? If you will recall how close Soarin' puts you to the screen, you'll know that pixel visibility would be disastrous.
With all that said, I haven't been to the park in almost two years, and I'm not even sure I got the chance to ride Soarin' on that trip. If in fact the film print(s) has been allowed to degrade due to improper care, or is just being presented dirty due to lack of proper cleaning, then that is truly sad. But, there is no viable digital alternative, unless Disney wants to invent a new camera and projector system just for that ride. That includes IMAX digital, by the way, which is only IMAX in name, and does not represent anywhere near the same quality, using two "2K" projectors for an effective resolution of 4000 pixels horizontally.