Do you think it is possible that cellphone cameras are coming closer to APS-C and thus its those that end up losing out? Full frame seems to be a significant value add relative to APS-C for a prosumer, just as how medium format is a significant jump up.

I think not. The difference between cellphone and APS-C cameras is in both quality and category. A dedicated camera has an ergonomic shape, a viewfinder, interchangeable lenses, hardware dials, switches and buttons. The people who don't care about those things have already switched to smartphones, while the people who enjoy photography as a hobby and the professionals will never want to give them up. The sensor of an APS-C camera is 20x as large as a cellphone's, so you're already well into the region of diminishing returns. Is a full frame sensor that is 30x as large really that much more compelling? Especially when you consider the downsides that come with supporting a larger sensor (larger camera body, larger and more expensive lenses, etc.) Maybe it is now, but advances in sensor technology continue to make APS-C good enough for more and more purposes.

Medium format cameras will still have bigger lenses compared to FF, regardless of the body size, due to the sensor size. This usually the show stopper, since lots of people have had FF DSLRs with the extra battery holder, etc.