The airlines don't have a stellar record of bringing them to the destination alive, for starters.
Are you going by the Karen's screeching on Facebook or do you have stats to back that up? I do... or more specifically, the FAA does. Here's the number of pets killed, lost, or injured by the airlines in 2020.
So, 6 deaths out of 311,149 pets transported is a death rate of 0.00001928 or 0.001928%.
But, you say, 2020 was Covid! Let's look at 2019 here...
That's 11 out of 404,556 which comes out to 0.00002719 (0.002719%).
I don't know what your definition of "having a stellar record" is, but I think you've set your bar a little too high.
The Facebook activists came out in 2017 because 40 out of 506,994 animals were killed, injured or lost and 31 of those were on United Airlines (18 deaths). What the Karens there didn't understand (and the media didn't report at the time) is that United was the only airline that would transport a brachycephalic (short nosed) breed. Those short nose guys are very susceptible to respiratory problems and there's no way you should be flying with one, especially as excess baggage or in a kennel underneath your seat.
Here's the 2017 data:
So, I think the airlines have a really descent track record transporting pets as excess baggage.
Especially in summer, there have even been cases where airlines (I think it was Delta) a few years ago even put a hiatus on animal transport because of heat related issues.
Isn't that what we want? Most airlines have embargos during the summer on transporting pets, either altogether, or out of certain airports (PHX, LAS, etc.)
Think about when they're unloading the cargo, on a hot ramp, etc. No grass around to relieve the dog, sometimes they do walk them but on hot pavement that's even worse when it burns their paw pads.
Source? From what I've seen the airlines take great care of the animals.
That's a stressful event for any dog, a dog who's already basically into a "hospice" type situation doesn't need additional heat and other stresses.
I agree that flying a sick dog who is that poor off should not be flown in the hold, but for a healthy animal, there is a greater danger driving to the airport than dying in the hold beneath the airplane.