The Expedition’s days are numbered, but it will not completely die off, at the very least, it may get a replacement. The "Expedition" nameplate may end, with the vehicle getting a new name. Happens all the time in the automotive world.
Until then (if that happens), the P552 is currently under development, and while I can’t say for sure if Ford plans to do so, the Expedition may start sharing sheet-metal and major interior components with it again, to cut costs. They have always shared chassis designs, certain suspension components, engines, transmissions, and so on.
The U222 back when it was released in late 2002 was the first Expedition to show differently styled sheet-metal and interiors.
It is confirmed that GM is readying its all-new generation Silverado/Sierra pickups, and the SUVs to be based off of it (Suburban, Yukon, Escalade, etc.) So a competitor is needed.