In abstract, yes. But the problem with the semi-flex tickets in particular was that they were poorly understood by staff and there was a not-very-clear 1 hour rule that meant that calling in the change advantage was very heavy weather. Now if a customer goes to customer service it's quite simple:
HBO: it will cost you a lot (and yes you can add a bag online or at the airport, also for a hefty fee).
Anything else: it will cost only the fare difference, which could be nil if the flight is low loaded.

It also gets around the silly situation when people are forced to scoff all of the lounge biscuits when they arrive early from their meeting, in order to sit on their prebooked completely full flight, when the earlier service runs out half empty. Which has knock-on benefits to anyone on standby for the later service.

Incidentally, I bet the HBO fares will continue to show up on the Flight Comparison websites such as Skyscanner. From discussions with BA managers, that is area where it really matters, and if anything I suspect BA's flights will undercut both LCC and legacy flyers for much of the time, as they do now.