They have not
always said that destruction would be for MP only. They have said that the power of the cloud would allow an amazing multiplayer experience, and then as far as I know haven't talked much about the Campaign (which is co-op, thus a "multiplayer experience", thus fairly to be assumed to have destruction in it; the E3 2014 version was even described to have "
open-world cooperative gameplay supported by dedicated servers", although I don't have the original press release to know what was paraphrased there) other than that Sumo was handling it and that it'd have some of the same stuff of Crackdown and cool new stuff.
They never came out and said, "This awesome destruction gameplay will be all yours in Crackdown 3's amazing Multiplayer Mode! ... And then also there's this naff single-player campaign that you can throw cars around in like the '07 game and some other simple jumpy-shooty stuff but don't get your hopes up for everything you see here. Pre-order now!"
Crackdown 1 previewed/demoed much worse than it played in full, so I'd love to be proven wrong with a trailer-defyingly awesome game...