If someone was willing to invest into building and maintaining infrastructure there would be no need for this concept, but that's a political issue. The idea of this concept is do make the best of what you have.
Of course those exist, but that does not make them standard compliant. Same goes for power supplies that don't do handshakes. That's not an issue if you exactly know what you need them for, but they're not meant for the general public because you can use them wrong.
Theoretically you could build a male to male contraption from multiple adapters and a cable. Also you could be providing too much current to a device, however this is specific to the combination of adapter, cable and power supply you use.
I can't quite figure out what would be the use cases where bcache would excel, except for hdds without cache or systems with very limited ram. Can you help me out with that?
Hmm the prices on eBay really don't seem that good to me in comparison to newer generations. I guess the power consumption will eat up any savings pretty quickly. Though this might differ for whole sets.
In my experience, WhatsApp does not send an image unchanged. So it doesn't matter if the recipient can handle it, as long as WhatsApp's conversion can