I don't question the decision at all. However, I'd like to point out that although the aggregate computing potential of the pi 4, while being potentially greater (6 core-ghz, vs 5.3), a working mac mini of that vintage is likely to be equally fast for use case of being a backup. It would be hard to figure out how to make parallel the tasks associated with copying changed files from one computer to another over a network, and the main limitation on speed will be the network connection rather than computing anyway. Furthermore, the pi being RISC based, may get less computing done per-cycle than the intel chip.
(I say this without biasing having had a techincal job in one of the two university computer science departments most responsible for RISC computing).
And the last bit of curmudgeonliness, is (excusing the irony blindness), I'd have used the term "enjoying" or "reveling in" reverse-sticker shock
instead of "suffering".
But *BRAVO* in any case.