Your point certainly sounds reasonable, but it's not how the human mind works. The point of conspicuous consumption is to make believe that the man is "in a position to throw away money like this", because that (imaginary) man is considered to be a good provider/sexually attractive. Talking about it beforehand would be like telling your kids you were going to dress up as Santa Claus to put presents under the tree—the reality is no different, but the fantasy is ruined.

And DeBeers certainly didn't create the notion of conspicuous consumption as a romantic gesture—women have been receiving silver and gold trinkets for millennia, especially ones adorned with various gemstones. DeBeers just popularized diamonds as the gem to be specifically associated with such gestures—and they're a fairly effective/obvious one, too, since they're eye-catching, distinctive, and easy to adorn oneself with.