Film cannot compete with digital projection.
As I've mentioned in other answers, "resolution" does not equal "quality." There are many things that go into a viewing experience which ultimately affect the perceived and actual quality of a cinematic viewing experience.
If the choice is between a 4K digital projection format and a film format (any film), digital will win with no effort, 100% of the time. There are many technical reasons for this, but the main ones are:
Steadiness of picture - digital projectors have no moving parts, and stabilizing a piece of 70MM film moving at IMAX speed is a difficult challenge requiring a complicated "rolling-loop" projection mechanism. The reality is that, even with all of this complicated stuff, it's impossible to ensure no movement from frame to frame. This degrades the quality of the image.
Dirt and Debris - film collects dirt as it travels from the platter to the projector and back. As it is engaged in the projection aperture, it also tends to shake off debris and little pieces of polyester and emulsion, which accumulate on the lens and other areas. I have no direct experience on how bad this is on IMAX systems, but it required pretty regular cleaning in 35mm systems, and I suspect it would be worse on a larger format. This dirt and debris accumulation on the lens reduces the contrast ratio of the projected content. Digital projectors do not have this issue (though some do get dirty over the course of years and have to be professionally cleaned).
Color - while I don't have direct understanding on the quality checks that they use for IMAX film prints, if they are anything like the 35mm checks, the answer is "practically none." While we'd all like to think that color rendering is consistent from print to print, the reality is that there are going to be slight variations in chemical makeup from one end of the reel to the other, and sometimes it just plain gets messed up (I've seen minutes at a time of footage that was totally off-color). Digital does not suffer from this issue.
Resolution - people throw around the supposed "resolution" of film like it is somehow equivalent to the square pixels we are used to in computers. Film does not work this way, so it's an apples-to-pears comparison. Additionally, there is a very finite limit to the resolution we're capable of experiencing, and it's measured in arc-minutes. Any resolution beyond this is waste, and it happens to be that around 4K resolution is the right number for a large format auditorium like IMAX. So even if the supposed resolution of film is better, this does not really translate to a better viewing experience.
Film grain - Film has something else that digital does not: grain! And the process used to transfer and post-process the original, and then later strike copies on film, results in degradation of the image in each step. I have never noticed the pixels in a digital auditorium, but I very clearly notice the film grain in a 35mm or IMAX film presentation. It's the biggest advantage digital has in my view.
Optics - a 70mm film print takes a huge aperture. And, a huge aperture requires a huge piece of glass to be able to focus the image from the aperture to the screen. From a physics standpoint, the smaller the source, the easier it is to focus it. The economic, physical, and technical realities of this mean that it is inherently harder to get a crisp, sharp image in focus on all corners of the IMAX screen. Digital projectors have smaller source sizes (0.98mm, for example) so the optics have a little easier time focusing the image. I don't know how much difference this makes from a practical standpoint, but it is a consideration.
Bottom line: Overall, there are several factors to consider when evaluating digital vs film presentation formats. Anyone who tries to make a claim that digital is somehow inferior to a film presentation in any real-world projection booth is suffering from nostalgia. I'd lump them in with the folks who say phonographs are superior to 96k audio. It's just not the case.
I too worked with film, and am very happy to be rid of it. The first time I watched a movie after converting from film to digital, my breath was taken away. It was a night and day difference, and we ran (we thought) a good film operation. Film just can't compete on any level with digital.
