I think you're underestimating the cost quite a lot. It's not just a piece of plastic and a few bits of wire, you also need a high performance I/O controller to go with it. Assuming SD card readers aren't massively overpriced, you're looking at closer to $5-$10.
Then there's the space considerations. If you've seen a teardown of a modern camera, especially mirrorless, you've seen that they're quite densely packed. It's not necessarily trivial to find the space for an SD card slot.
I agree that if it were essentially free that more cameras should have them but I'm not so sure that it's as trivial to add them as you make it out to be.
> Arguing that nobody "needs" reliability until profits are on the line isn't a particularly compelling argument in my opinion.
My argument isn't so much that nobody "needs" reliability as due to modern reliability, few people would use it if they had it. It's very rare to hear about a dying SD card so it's not something an average person considers when buying a camera. Even if cameras had dual slots, I doubt your average family going on vacation to Disney would think to buy a second SD card for it.
Since few people want it and there is a cost for it, it makes sense to get rid of it.
And they're all $1200+ cameras (with no lenses). That's a substantial price for a feature that has become cheaper to add over the years. An SD-Card bay is under $2, the plastic bespoke door that covers both bays likely costs more than adding the second bay would.
Strikes of artificial price segregation, the whole "prosumer" tier didn't even exist back in the mid to early 2000s.
> The only people who need extra reliability are professionals who lose a great deal of money in that 1 in 1000 case where an SD card dies.
Almost all photos are once in a lifetime. That wedding, birth, or even vacation likely won't happen again (and if it did it would be different anyway). Arguing that nobody "needs" reliability until profits are on the line isn't a particularly compelling argument in my opinion.