imagine vertebrates never developed. Would nature have to invent them, or would some other branch of life develop to fill that niche?
Well, you're not going to get very far with any form of industrial civilisation without some form of rigid structure to manipulate the world with. And there are straightforward physical reasons why exoskeletons stop being practical beyond a certain size, and so only the endoskeletal lifeforms continue to larger sizes. Symmetry leads to a natural preference for even number of limbs, and four is an obvious minimal number for ease of locomotion (six to provide for four on the ground and a pair for manipulating things would also seem reasonable, but coming through an evolutionary path like ours, there's no obvious way to get them at the point that the need for the manipulating limbs appears). Having a movable top to hold visual and audio sensors is also an obvious evolutionary advantage. So consideration of the underlying needs and physical limitations does seem to point to the development of a structure not unlike out vertebrates.
Most of those features are okay; they're almost required to "fill that niche". I'm not sure about those handwaved "physical reasons". Coconut crabs can reach six feet, which is probably sufficient size for an industrial civilization.
(You may be thinking of the limits of the trachea, perhaps? With branchiostegal lungs, the coconut crab has transcended that limit. If the vertebrates were to disappear, the coconut crab would be a good candidate for evolving into species that fill at least part of our niche.)
But usefulness does not a requirement make. Movable top? Useful, but not necessary.
Vertebrate features that are not required by the "industrial civilization" requirement include bones, teeth, hair (as mentioned), jaws, mouth breathing (no invertebrate with "lungs" breathe through their mouths), and necks (hey, if you develop from a species without vertebrae, you're not going to suddenly have a few vertebrae just because a movable head might be useful - see insects for one or two alternatives). And that's just touching the head ...