That is the Sunk Cost Fallacy, not some dog who’s too stupid to stop digging.

Sorry, but you are wrong. You are assuming Sunk Cost Fallacy only applies to "“throwing good money after bad”. It also applies to time out of one’s life. That is why it is often assigned to people who stay is relationships for longer than they should. The dog is throwing time away and refuses to waste the time he’s already spent. See, that is the same as your example of spending an additional $400. Sunk Cost Fallacy applies to time as well.

he has absolutely zero expectation of actually getting a bone if he keeps digging

How do you know that the dog has zero expectation? You only know this because you are seeing a zoomed out view of all the panels of the comic. That is not the same view the dog has and the dog does in fact expect to find the bone if he digs just a little more.