I can assure BOB does not take anywhere near as long on the huge majority of flights.
Not everyone will require serving in the way they currently do as they are "entitled" to it. The crew could end up walking by a number of rows before actually reaching anyone that wants to purchase something. Of course the busier leisure routes will most likely generate the most sales. Again just like with any airline with BOB.
Other airlines manage it extremely well. Im sure BA will have a plan in place for busier flights behind the curtain, just as they do now with busier flights infront of the curtain. Infact its not the first time they have offered BOB. Ba Connect used to, albeit usually on smaller aircraft. Not to mention a wealth of knowledge and experience from behind the scenes from their IAG partner airlines and even colleagues who have not always worked with BA.