I'd say other reasons why this idea won't work (pax below decks) is the cost of staffing and cleaning. CC during flights to check and serve, extra time to check and clean after each sector, more laundry - can they charge enough? But the safety issue and the need for pax operable emergency exits to be placed in the hold - will stop this.
The emergence of Suites in all the top carriers have bypassed this idea and some carriers are reducing the number of F seats anyway.