Your upstream needs to be able to handle 100% of the load, which could be several orders of magnitude more traffic than it normally has to handle. Handling that

Your upstream needs to be able to handle 100% of the load, which could be several orders of magnitude more traffic than it normally has to handle. Handling that huge spike in traffic for, what might not be more than a few minutes a year, can be insanely expensive. Also, you're potentially giving users a noticeably worse experience as you rebuild the cache.