CAFE standards, 10 speed transmission, smaller displacement boosted engine...the writing is on the wall. Would I like to have a twin turbo or SC'd coyote? Sure, but it's not going to happen unfortunately. Take rate for the 3.5 over the 5.0 in the F-150's is currently higher as well.
No reason for Ford to offer a V8 in the Raptor going forward, unless it doesn't sell. Given F-150 sales only get better every year, I have my doubts that it won't sell better than the Gen 1. The next GT500 may even have the 3.5 in it.