A dog that gets constant and regular heartworm treatment does do more harm to the dog than good and yes regular heartworm treatment can and does shorten a dog's life.
A dog gets heartworms when they are bitten by a mosquito and if the dog hasn't been bitten by a mosquito then there's no need to treat the dog for heartworm.
However if you know your dog has been bitten by a mosquito then it would be a good idea to get the dog treated for heartworm just in case the dog has contracted heartworm.
It's best to have the vet treat the dog for heartworm using an injection and they only need the heartworm treatment injection when they get infected or potentially infected with heartworm.
Regular heartworm treatment and heartworm treatment does come with risks and can shorten the dogs life so it may not even really save the dog from heartworm and may do more harm than good.