Birds don't "just die".  They die from some cause or another and those causes should be managed so they don't take the lives of birds.
It's a very rare chick that is shipped with a birth defect. Mostly, they are leg and beak deformities where a decision is made to put them down.  (and shame on the hatcheries for shipping them in the first place)
People who practice proper sanitation and feed a balanced diet should not be having disease issues.  Disease doesn't appear out of thin air.  Maybe someone somewhere has had a wild sparrow infect their flock with something deadly, but for the most part disease is a management issue.
It's poor animal husbandry to order chicks in the summer if you live in a place that gets too hot for them to survive. If it is hot, then measures should be taken to keep the birds cool enough so they don't die. Lots of poultry lives where it is hot and they survive. Heat is a management issue.  Lots of people get their flocks through heat waves without losing 30% of their birds.
If the weather is cold, you supply shelter and heat. My birds have survived 50 below with no losses; don't try to tell me that poultry can't be brought safely through cold weather.  Poultry survives in Canada and Alaska. If a person lives where it is cold, then they really ought to have measures in place and ready to deal with freezing weather.
There is a ton of information available about raising Cornish Cross.  Home raised birds should be better raised than commercial birds, and the commercial poultry farms don't have anything near 30% losses.  Many of those farms are located where the summers are hot and humid.
The only Cornish X I ever lost was due to very poor management on my part. She drown in a water trough.  Well, mea culpa, I should not have allowed her access to water that she could get into and not get out of. 6 month old Cornish Cross hen, healthy as can be, but apparently not such a good swimmer.