I really like that idea and I don't want to suggest that it can't ever work, but the sudden purchase frenzy that did not happen is far from the only failure mode. Leave it running a decade or two and things might look very different. Slow, invisible expenses creep and in the end you have a culture where those who spend a little slower than others are shamed into buying stuff just to make the others not look wasteful. There are many subtle mechanism at play that all facilitate expenses creep and there needs to be something pushing in the opposite direction. I'd rate it an unsolved problem.

Perhaps some generous budget limit, with a clear message that it's not intended to be fully consumed, and a clear rule that a certain percentage of whatever will be left at the end of the year is donated to some universally accepted good cause? Children and cute animals, please not religion. "You can get the book, no problem, but keep in mind that being the book means $5 less ponies!" (just don't make it manatees, you'd risk me working on a chromebook)