Your expectation of a price reduction following the introduction of BOB is somewhat naive. That is not how airline tickets or indeed many perishable commodities are priced. Prices are set to compete in the market place and fluctuate all the time. The cost of goods or services are never the sum of their cost in a market economy; it is what the market will bear. Indeed BA sells many tickets that won't cover its true costs.

BA communicated, indeed promoted, their new shorthaul Y proposition, which is as much as a consumer can expect. As you know there are other options, ie BA's competitors. If passengers flock there, BA will revisit their approach. But I'm doubtful that that will happen. I think it more likely that others will follow BA.