Rotties, Akitas and many other mastiff breeds can be just as big and muscular and/or do just as much damage, nobody is disputing that. If you're being told by people who study dog behaviour and train dogs for a living, professionals who have worked with all of them, that the Bully XL(the UK lineage in particular) is standing out. With examples and factual statistics to back that up and they are refusing to work with them, then why would anyone ignore that? They are more human aggressive and extremely over reactive, in comparison to any of the breeds people are trying to compare them with, so maybe it's not worth trying to make those comparisons as any sort of defence for the breed. It sounds like they are very hard to train and there are genetics making them predisposed to certain traits at play here(hence why at least one dog behaviour expert has been killed by one of these). Maybe some of these dogs don't have these genetics but quite a lot do(too many) by the looks of it and how will know you picked the short straw until you find out?
Though it is true, a lot of the owners of this breed will be gangs and insecure thugs, some of the owners in these videos circulating, seem to be middle aged women and men and the likes(every day people) struggling to have any control over their dog. Given the above facts, it might not simply be that they are bad owners, who haven't attempted to socialise them etc.
It is true the gangs and thugs will move onto another powerful breed and some of those will be brought up to be human aggressive and out of control. So maybe it is time that is addressed too. But lets not act like this is a new phenomena either, so that simply can't explain the figures we have seen in the last 3 years are like nothing we've seen before.
I'm not actually calling for all of them to be destroyed, just have the owner come forward show they are suitable and willing to accept full liability, agree to keep it muzzled in public(why moan about this, when the alternatives are much worse) and so on. Maybe even have the dog checked over by a dog behaviourist, to see if it's one of the ticking time-bombs, if they are sure it is then neuter it definitely, I don't want to say euthanise it because surely there is better solution if it has done no wrong. Though I'm not sure what that is(don't keep it in a family house? make it a working guard dog... having a purpose like that normally helps). All of that, sounds like it would take too much effort and too many resources unfortunately though, to be fair.