Note there is an icon field, this is how the icons are specified for navigation entries.
It's certainly true that add-on related data is usually uneditable; but there are aims to make certain aspects of some add-on data more customisable without those changes being overwritten by upgrades. We could, in theory, allow editing of the icon here.
That said, even if we don't, we're still talking about more of a styling concern here. So it's true that there could be a style property to control this, but even if there eventually isn't, you would just remove the icons using CSS, a vague example:
It takes a matter of seconds to switch all FA icons off in the navigation bar using the CSS I mentioned above, this would likely be your best option if you're wanting to disable all icons in the navigation.
For a number of reasons, I think it would be unlikely we'd have a switch dedicated to switching off just navigation icons.