I don’t know what the answer is, I do know that the dogs that cause the biggest issues at our local park are rescue dogs that have been imported, maybe stopping that would be a start.
To give an example there is a middle aged couple who had never owned a dog before, they took on a six month old street dog from Greece, at least they were told it was six months but the rescue had no way of knowing. They were told it would be a medium sized dog, it has grown taller than a Labrador and is extremely strong. Unfortunately it is also reactive to dogs and people in the last few months it has bitten one man and lunged at another who narrowly avoided being bitten on the face. This is despite it always being on a lead.
I was taken off my feet last winter as these inexperienced owners kept it on a long lead and allowed it to run behind me, I had a deep black bruise on my hip for several weeks which would have certainly meant a broken hip for many older people who use the park. Many of us tried as experienced dog owners to suggest training classes etc when the dog first arrived, but they said they had been advised that they had to do that as a couple and as the husband worked in London they couldn’t do that.
Yesterday he lunged at two dogs including one of mine, despite knowing what he is like his owner still wants to come over and join our group of little dogs.
One dog, one park but as PP said a combination of inexperienced owners and the wrong breed, but I think street dogs with no history has increased the likelihood of that combination.