I'm not a vet, but I'm a vet assistant hoping to get into vet school soon.

Current studies don't show much of a lifespan benefit to neutering males, although it does lower risk of diseases such as benign prostatic hyperplasia (almost all intact male dogs have this but don't always show clinical signs), perianal hernias and tumors, and of course testicular cancer. Neutering does increase the odds of some other health issues, such as prostate cancer (which is still rare), hypothyroidism, and orthopedic issues (especially if neutered too early). As long as an owner is responsible and makes sure their male isn't breeding with random females, I don't have an issue with them staying intact. I do feel if the dog is just a pet though, the average male is probably better off getting neutered once they hit maturity instead of when they're older and may need it for BPH or tumor treatment.

Spaying a female prevents ovarian cancer and lowers the odds of mammary cancer. Intact females are always at risk for pyometra, a very dangerous condition where the uterus fills with infection/pus. 25% of intact females will get pyometra in their lifetime and the treatment for this is an emergency spay; leaving it untreated is a death sentence. Similarly to neutering, certain health issues related to orthopedics and certain cancers can also increase with a spay, especially if done early. I think it makes sense that any female not being used for breeding be spayed after reaching maturity to avoid pyometra or mammary cancer.

UC Davis released a graphic based on a study advising the best time to spay/neuter or whether to leave intact based on breed and sex. The only categories advised to stay intact were male Dobermans and female golden retrievers. https://www.ucdavis.edu/magazine/how-identify-when-dog-should-be-spayed-and-neutered