This is one of the most brilliant aspects of the deBeers coup - it tapped into the very wide-spread and long-established custom of the bride-price!
In cultures where this custom is still practiced, every young girl hopes for a high bride-price, which not only brings honor to her family, but gives her confirmation that she is special, that as a commodity, she has value. She is such a high quality product that the groom is willing to pay a premium to obtain her.
deBeers managed to tweak this to fit the rapidly changing societies of the west, where the vast majority of young people were now choosing their own marriage partners, with little or no help from mom and dad, and associate bride-price with romantic love - with the diamond being the symbol of how much the the prospective groom loved his sweetheart!
And so today, we see young women who, as you point out, are frequently financially independent, and many of whom may have little or no knowledge of the ancient custom of bride-price, much less any idea how ingrained its essence is in their own cultural values, who sincerely feel that a diamond, and as large and costly as possible, is the engagement ring of choice, and for some, inability or disinclination of the groom to produce such a trinket could be a deal-breaker, and they may say good-bye to the man for whom they have the greater affection in favor of the suitor who promises a ring of the desired price range.
Of course, it would not be a question of them holding out for the highest bidder, rather being pragmatic and realistic about future financial security. Being mature.
And so, even a discussion of diamond versus cz brings us back to that fundamental question:
What does marriage mean to you? and reminds us that even the most modern young women, even among a prevailing culture overwhelmingly presenting itself as overwhelmingly all about romance, and romantic love, when it gets down to making that commitment, for many of our most sophisticated and urbane brides-to-be, old ways are best, and nothing will seal the deal like that sparkling affirmation of her value as a commodity: a high bride-price!