Lots of you would be horrified at the mutt in our family that was rescued from Romania, dumped in a forest. She is the sweetest, kindest dog that I have ever met, so good with the young children in the family and so happy to be in a home
While our Romanian rescue, who was in private care from a pup, is entirely unpredictable, with significant aggression issues. It's the risk you take if you rescue a dog with a complex background.
Some pedigree dogs are aggressive. However, when choosing a pedigree, you can thoroughly research the breed characteristics and the temperament of the parents - a good breeder will be aiming to prioritise temperament, especially in a pet breed. A good breeder will also raise the puppies in a suitable environment for their future, and then it's the owner's responsibility to ensure that they socialise and train the animal appropriately. Once you allow for these factors (i.e. not counting foul tempered animals bought from crap breeders / owned by idiots who solicit poor behaviour with bad management), the number of temperamentally unsound pedigrees will be much smaller.
If you're lucky, a rescue dog will have all those advantages and be a lovely dog. If you're not, you'll be stuck dealing with someone else's mistakes, having missed the most important stages where you can influence a dog's development.
People absolutely should rescue dogs if they're in a position to give them a suitable, safe home. Many of the dogs in rescues, however, are not suitable for the average household with other pets / children / people coming and going, and their existence should absolutely not prevent good breeders from producing and selling more suitable animals.
The caveat to that is that there are some breeds I'd be happy to see junked entirely - pugs, for a start - while others could easily be bred away from absurd breed standards, towards a healthier version of themselves.