Let's face it: APS-C was invented in a time that full frame sensors were very expensive. Now that that problem has been solved, who needs APS-C?
Yes - completely agree with this point as well.
I think we need to really think about the broader photography industry as not just limited to traditional cameras, but also cell phones.
Cell phones have taken over much of the point-and-shoot business, and as photography applications become more oriented to web applications (i.e. Instagram, Snapchat, etc.) there will be more and more people using cell phones as opposed to cameras when travelling. Cell phone cameras continue to get better for the "all-in-focus" type of shots, and are developing more software solutions to replicate key features produced by traditional cameras, such as bokeh.
This means that traditional camera companies must now further widen the gap between cell phones and traditional cameras - make the technology better, including faster speeds, more megapixels, better resolutions, etc in order to attract more casual buyers. I just don't see APS-C continuing to survive in this broader market. This is especially the case when a basic entry-level APS-C with basic-quality kit lens often produces a photo that is no better than a photo captured from a modern cell phone - sad, but true. You'll never see this issue on a FF.