I come at this issue unashamedly from the perspective of rescue. I think it's worth standing back a bit and remembering the 1970s and 1980s and the norm back then was for dogs to be un-neutered. It was also the norm for many thousands of stray dogs to be killed each year. Since then, a mountain of studies have cited the effectiveness of large scale spay/neuter programmes in reducing the death toll and the population of stray dogs and gradually, a culture has developed where spay/neuter has been normalised and seen as the responsible thing to do. Most rescues insist on pets they rehome (provided they are old enough) being neutered.
There has also been a big shift in society's attitude to dogs as a whole (you only have to look at some of the threads on here regularly). There is far less tolerance (rightly so) for stray dogs, and aggressive dog behaviour - entire male dogs are over-represented in studies of dog aggression, both dog on dog and dog on human.
Entire dogs do wander after bitches and this makes them vulnerable to straying, to car accidents and to other harm.
Looking at the medical issues, yes, there are some slightly raised risks for some breeds. However, the impact of neutering is tiny compared to the horrific premature death rates for some breeds exacerbated by
adherence to ridicolous pedigree standards, artificially small populations and high COIs. I'd love to see vets (as a body) speaking out much more loudly about some of these issues.
Having your pet neutered also protects against theft. Dog theft is on the rise and an entire male from a popular breed e.g. Lab, poodle, Bichon etc is worth £££ to unscrupulous BYB/puppy farmer.
I think it's helpful to consider the Swiss Cheese Model from Health and Safety (most of us will have seen this on training courses for risk assessment etc.). Yes, of course, highly responsible owners will ensure that their entire dog is carefully and thoroughly trained, always wears ID, has a chip with up to date details, is never left where they could be stolen, never has any access to means of escape, and will never encounter a bitch in heat.
For those of us who are more risk averse, neutering is part of a package for responsible dog ownership. Our dogs all wear appropriate ID at all times, go to training classes and we exercise them responsibly and live in a house with a secure garden. But because we're human, and I can't guarantee that other people are also responsible, we do everything we can to fill in the holes in the cheese. For me, neutering is part of that.
I am very much against early neutering however, and think that there are some serious concerns about the practice of neutering puppies and very young dogs, even more so for larger breeds. I'd like to see some of the bigger rescues who routinely neuter puppies such as Many Tears and RSPCA talk about the health issues around this. I remain to be convinced and agree that sex hormones play a crucial role in growth and development.
I should also add the rider that in rare cases, vets will advise against neutering on medical grounds for an individual dog, and that of course is entirely appropriate.
Sadly, I can see this issue going the way of vaccination, particularly as many of the vets and campaigners who are currently promoting the no-neuter message are usually also anti-vax as well.