For example, I'm a sailor and a navigator. I would be very helpful in a community that relied on fishing and maritime trade (or piracy!). I'm sure I could find or improvise the tools, reference books and instruments I needed for navigation, and teach the youngsters how to get along without them after I was gone. I could even make a compass out of an old loudspeaker. But I would have to depend on others for those skills necessary to keep boats in the water.

I don't know how to work with fiberglass, I have no small engine maintenance skills, (and I can't refine gasoline!). I can't weave sailcloth or make rope, and I have no carpentry skills to speak of. And even if I and my community were able to put together a small fleet, or even one seaworthy vessel, what would we have worth trading? And could we catch enough fish to feed ourselves without a knowledge of fishing that did not depend on now-unavailable technologies, like line, hooks, nets and so on? And although I am familiar with my local waters and the marine life, I don't even know how to preserve smoked mullet, one of my favorite seafood dishes!

In fact, I toyed with this very scenario as a plot for a novel, and soon came to the conclusion that there was no way I could convince myself it was workable, much less persuade my readers that it was even possible. And of course, I have no idea how to organize the social collective needed to provide and assemble the total ensemble of skills needed to supplement each individual's skills. I don't think I have the leadership skills to make it happen, either. Being a good boss may be the hardest job of all.

Technologies, even the simplest and most primitive ones, are not things. They are systems. They require a pre-existing infrastructure. Each technology, be it fishing, hunting, farming, herding cattle, whatever, is the result of a long evolution from simpler systems. The tools and social organization needed to implement these systems can be improved on, or built upon, but it can't be raised from scratch overnight. Occasionally, a culture can adopt a new way of earning a living from a neighboring culture, the way the Plains Indians learned horse tech from the Spanish, but they already had a functioning and successful culture that could keep them alive for a few generations until the new tech was mastered and integrated into their environment.

You're right, it takes a village. This is why I am betting on places like India. A village can raise food and make tools, with un-powered machinery. It has potters, weavers, carpenters, a blacksmith; and it can trade with neighbors who might harbor a tanner, a harness maker, a wainwright, a cooper, a work animal tamer, and so on. The abandoned cities could be mined for raw materials. In fact, in remote agricultural villages, life might change very little. This is how I visualize the landscape of Gaul and Britain after the Legions went home. It probably wasn't all that different from the time before the Legions first came.