"Keeping a potentially dangerous dog alive is completely irresponsible. I just can't understand why people do it. It's crap for the people, crap for the dog and a disaster waiting to happen."
If someone is unable or just doesn't want to manage and live with a dog that has a behavioural issue that could be dangerous - then absolutely having them PTS is the best option IMO.
But, it isn't irresponsible or a disaster waiting to happen if the owner is capable and willing to manage and work with the dog.
Using mine as an example, apart from 5 - 10 minutes of the day, he's a happy, well behaved and much loved dog.
It's been 2 years and 4 months since he first showed aggressive behaviour...he has never bitten a person or a dog and in fact has never come close to it after the first two weeks of him showing that behaviour, because I muzzle trained him and control his interactions.
I would never willingly have taken on a dog with the issues he has, but, by the time they developed...he was my dog and aside from me having a responsibility to do right by him, he's my dog, I love him.
I constantly assess whether his quality of life is ok, and whether I'm still ok with owning him because when either of those is no longer true, he'll be PTS.