Its a layered world we live in - i.e. complicated. On the one hand, there is a definite trend to provide smaller mobile phones, computing devices to folks, esp. the younger generations have acclimated to these cultural changes. Indeed, the smart phones have decimated PS cameras, and it hasn't stopped there. One FF couple i know of (he shoots birds with FF and a 800mm lens, she shoots landscapes with a D800), bring their Iphones on trips, they shoot landscapes and videos with the IPhones, and the FF cameras are used for longer critical shots. She recently went to Africa on a safari tour, taking only an iphone for general shooting, a D800 + 70-200 lens for the safari wagon. Multiple FF lenses were eliminated by taking the iphone. She is petite older lady with health problems.
Most FF cameras, that i know of, will let one shoot in crop fashion as well as FF. If you buy a FF camera body, and only FF lenses, you're talking about a lot of money, a lot of weight and a lot of volume for the pack. My plan, is to buy the smallest FF body i can (i.e. the A7XX maybe), no more than 2 FF lenses, use crop lenses from my Nex - cheap and light for walk around, use my Pentax macro, and other Pentax lenses with FF camera because its so good with manual focus, and yes, i'll be keeping my K3 and my f2.8 theatre zooms for that niche. The beauty of my current strobist collection of manual flashes, including the Cactus v6 radio triggers, is that they work equally well on both my Pentax K3 and my Sony bodies with the standard hot shoe. Yes - i've tested them.
My plans are still developing, but my main point is valid. One isn't restricted to one brand (or sensor) of camera/lenses anymore. You can spend your money for the shots you really care about and use smart phones (or some other sensor size) for the rest of it.
Linear thinking where you can only use one kind of camera, one sensor size, everything has to be DSLR, or one make is obsolete. Think of my club members i wrote about above, they are combining smart phone pics with FF pics. I don't know if they were just lucky or skilful, but Sony has tapped into an undercurrent of photographic desire not to be limited to one make of lenses. Am i just dreaming with my FF plan - nope. I currently shoot weekly with both my Sony and Pentax cameras - its doable. I can mix and match.
Entropy and chaos are the natural order - get used to it