You are enormously oversimplifying the conversations that happen over in The Doghouse.
The conversations around dogs with behaviour issues usually revolves around "see a professional". Or to expand - see a vet because many new behaviour issues are caused by a medical condition (often pain-related), and then get a referral to a properly qualified behaviourist.
If the vets and behaviourists cannot make adequate progress with the dog, that is the point to start discussing PTS - with the professionals, not with an unqualified set of randomers on MN.
No one over there is advising rehoming as a first port of call where there are behaviour issues - no one. Indeed, rescues tend to fall into two categories - the open intake ones (e.g. Battersea) who will take any dog but will put some down, and no kill ones (e.g. Dogs Trust) which are highly selective in their intake policies and will not knowingly take a dog with significant behaviour policies - but they will still PTS for severe behaviour issues. The trouble with the no kill rescues is that they effectively just displace the problem dogs elsewhere while polishing their own halos.
Behaviourists can work absolute wonders. My own dog developed a really significant issue with visitors after a house move - he wouldn't let anyone through the front door except me and my DF, and it was aggressive. I had to lock him in the bathroom if a tradesman visited, and couldn't even consider having friends over. We took one trip to the vet, who referred us to a properly qualified behaviourist, who solved the issue with one lesson and quite a lot of homework. A few weeks later, the dog was convinced visitors were there to play fetch with him, and we were conducting a series of lodger interviews. No problems since then.
Aggression doesn't come in the binary "aggressive" or "non-aggressive". A great many dogs look aggressive from the outside in certain situations, and are absolute gems outside of that. Mine is terrified of one particular breed that weighs 4x as much as him and will run up at them barking given half a chance. We avoid the breed like the plague, though I know there are a couple of owners of said breed that justifiably think he's an absolute twat. Outside of that I frequently get compliments about how well-behaved he is.
Most people with trickier dogs employ a combination of a behaviour modification plan (from a behaviourist), with some management techniques (like avoiding triggers).
The trouble is that when you're talking about a dog like an XL Bully, there's just no margin for error. If the dog is capable of dragging you down the street, and it has behaviour issues, there's no way you can safely put in place a behaviour modification or management plan. If the dog is aggressive towards the owners and is ultimately stronger than them then there's no feasible way to put in place a safe behaviour modification or management plan.