5e D&D functions really well with all spellcasting classes and subclasses removed. It can appear less satisfying because it leaves more than half the classes and a whole section of the book out of the game, but the game would be sound and the characters would perform as intended (I'd expect that in a no-magic world, the threat of demons, golems, and enemy spellcasters wouldn't be as present as in a regular game).
So while 2/3 of the pages would be removed, the system itself wouldn't be amputated of essential components. One could take the SRD, remove everything magical, and be left with a perfectly playable RPG. It's just that D&D fans would be missing a lot of things.
You can go a long way in refluffing many magical spells and abilities as mundane, and you can go even further if you are willing to tweak things a bit