we had a visit at school recently for guide dogs for the blind and they brought 2 dogs with them 1 was in training. and the other had failed due to the fact that he wouldn't walk through puddles. I asked what happened to failed dogs and she said most of them get kept buy their puppy walkers.
Ds's friend's mum trains guide dogs. I rang her up and asked her about this as I thought this would be a good option for us. She said they hardly ever come up, and the puppy walkers get first dibs. In fact she kept one who didn't make it (excessive barking).
y parents adopted a retired Guide Dog years back. They had friends who Puppy Walked and one of their 'walkees' went through all his training.....then it was touch and go because they found out he was diabetic. Though the GDA pays their vet bills , it was difficult to find an owner who would take on a dog with the daily needs of a diabetic dog. Doggie landed on his paws and found his new match- a retired Dr -who could recognise the symptoms even without being able to see them. And do the vital injections.
Ultimately it depends on why they've not passed their training. If it's a temperament issue it doesn't mean they're not suitable to be a pet dog, just that there might be a bit of extra work to do with them in training. If it's a health thing, or a problem with their toilet training (i.e. I know of one GD who failed partly due to coprophagia - i.e. eating its own poo) then it depends on whether you want to take that on.
All the same - if you like retrievers (Lab, Golden or a mix of the two - there are G.Shepherds too, and some other breeds, but the vast majority will be the retrievers) then I would say go for it. They're great dogs and all from good bloodlines, responsibly bred.