Consider the entorhinal cortex and what is being represented: a mixture of the vestibular system, visual representation, self-motion, postural feedback. Perhaps more. When you look at the repeating patterns where do you see any of that in the distributed dispersal that we choose to call a grid system?
I see that as the output of a common digestion system that ends up in a “common format.” The grid cells under discussion are a byproduct of explaining the coding that surrounds the hippocampus. Even though there are some very interesting theories to be best of my knowledge nobody really is sure of how that grid pattern is formed.
Looking at the dizzying maze of fiber tracts that connect the maps in the brain I have often wondered about the palimpsest of patterns that lay one on top of another in these many tracts. Consider the areas that receive the foveal vision as the eye darts over a scene, bit by bit - color, texture, stereo disparity, edges - all overlapping on the same maps. Within a few layers of maps that stew is mixed with the digested versions of body sensors (vibration, temperature, joint position and muscle stretch) to guide end-effector motion in space in the association areas.
The same general thing is going on in the auditory tracts.
Fat fiber bundles connect these diverse association areas and communicate some sort of meaningful information.
It gives me hope that if we can tease out how the wildly dissimilar information is combined in the grid maps that perhaps the same general process is working in everywhere in the brain.
You call grids a specialized navigation system - I see it as a possible general coding scheme that has been demonstrated to work in part of the brain. I will say it again - if the cortex is a general fabric that is uniform throughout then working out how part of it functions could apply to all of it.
Ask yourself - if you were going to take all of these wildly diverse sensory organs and apply a common process to turn it into a maximally dispersed (and compatible) coding to gain the best resistance to decay over time - what format would you expect it to end up as? For bonus points use a fabric of HTM style columns that do local processing to do this job.