To be fair, vets see the downside to not spay/neutering. They see the unwanted litters, and the rare dog who gets cancer, and the like, where the healthy ones don't make the same impression on them. And alot of the earlier studies insisted that there WAS no downside to early s/n. Except that the earlier studies were done mostly with smaller dogs, who reach their maturity ALOT younger and are less likely to have joint problems in hte first place....but its not that hard to discuss the negetive sides too, and I suspect that your average medium to small dog owner is going to opt for s/n fairly early ANYWAY rather than deal with a bitch in heat or the potential downsides of an intact male....
And I don't really blame the rescues and the shelters for insisting that the animals be s/n before they're handed off to their new owners, for much the same reasons. I DO wish that vastectomies and tubel ligations were more easily accessible and that the rescues and shelters would allow that instead of full s/n, but I don't blame them either.
