I know nothing about “capos” or guitars, but a quick search finds me a whole range of items so described, from $6-20, many with metal parts a printer would struggle to equal for durability. Even if we discount the materials cost for the printer, you’d have to use your printer to supply a need of a similar scale at least once a week for a few years for the savings to justify owning the printer. This just doesn’t happen.
There are other reasons to own a 3 printer: it’s a cool thing, it offers convenience, arts, hobbies, etcetera. And even if you can’t economically justify owning it yourself, it’s an interesting technology and great to have access to. You ask why a critical poster on this forum “is so certain [3d printing] can’t happen” - I don’t think many of us are certain of that. We’re just fairly certain the study being cited is utterly ludicrous; this doesn’t mean we think 3d printing is, or that it has no future. This sort of easily discredited puffery is not the sort of boosterism any idea needs; if anything, it hurts the idea, even if it’s a good idea.
