Sure, but you'd still need volunteers to run (and not cheat) their benchmarking service on the long tail of far too many hardware variants for them to purchase

Sure, but you'd still need volunteers to run (and not cheat) their benchmarking service on the long tail of far too many hardware variants for them to purchase and keep in a dark room somewhere, as well as some way for vendors to set the minimum requirements in a useful manner (e.g. ideally a one-dimensional scale per type of requirement), or it's not going to get used. I think it's certainly doable to some approximation of correct, and that Valve is probably in the best position of all the game distributors to do it with their existing HW survey infrastructure (and quantities of app on their platform) giving them a large background in what they're looking to quantify, and for whom. I just wanted to observe that it's far from trivial, and would still involve some level of crowdsourcing data (or paying someone like PassMark to use their single numbers for performance of given pieces of hardware as a scale for vendors to map things against, though the different workload numbers show that making this comparison isn't simple...)