We've been through the "monopoly" argument before. There aren't many major destinations to which there is no non-stop alternative to BA from London. Connections are usually optional even if you want to avoid BA. You're only "captive" to BA if you insist on flying the same airline all the time.

But then you have to ask: "Why would someone choose to do that?" The answer is usually to reap the benefits of frequent flyer status, which tend to offset any deficiencies in other areas of service.

Which is why people with high status levels who whinge on FT that BA's service is so bad that they're leaving are often to be found still choosing to fly BA years later, sometimes after "leaving" multiple times. (I see that the OP didn't need actually need welcoming to FT.) We never know the extent to which the grass was found, after all, not to be greener on the other side of the fence; or to which the benefits of BA status were, after all, found to be really worthwhile.

Sometimes it's coupled with apparently-authoritative predictions about how BA is about to bite the dust, yet somehow that's never happened yet either. Even with the pandemic, and even with the effective loss for the time being of BA's supposed advantage of a "stranglehold" on LHR slots.

BA isn't invulnerable, and it does have real and effective competition (which, in the real world, does include connecting itineraries at appropriately discounted prices). It could get things wrong and get into lots of trouble, as it has done at times in the past. Like any airline with a strong hub, it has advantages at its hub. But the idea that BA doesn't care and doesn't have to watch what it does because its passengers are forced to fly BA just isn't true. It probably never has been during modern times, save on a limited number of routes.