What are the very obvious downsides to using these, lifetime or cost?
This form factor has a huge advantage for OEMs - they can just slap on a single chip that has everything built in - controller, memory management and the actual storage. Less parts = less convoluted assembly process and that is cheaper. While theoretically you can put a BGA fully integrated SSD SOC like this on a PCI-E card or M2 etc., it kinda beats the purpose.
I did my early engineering years with DIP parts, the days of when you could just desolder the memory device and place with a higher density device are long gone.
That should be an absolute dealbreaker, though. Talk about holding your data hostage...
Also, get real, most people don't have any backups set up at all. They still shouldn't be artificially prevented from accessing their data in case of a failure...
Also known as eMMC.
If you can get your memory density high enough there is no need for multiple chips, if there is no need for multiple memory chips then a discrete controller doesn't makes sense since it only needs to communicate with a single chip in that case the extra die costs can easily be canceled out by cheaper packaging costs and the fact that you can now sell SSD's to system integrators that could not use them before.
We're not quite at the stage of putting the controller in the chipset and just soldering Flash to the board for PCs, although that is how it's done for SoC systems.