I've never bought a multiplatform game because of a marketing deal or even exclusive dlc honestly.
I have bought multiplatorm games that were outright exclusive for a period of time though.
It's more about getting the PS4 brand out there in advertising, you'll be seeing what are likely to be the two highest profile games of the holiday period and maybe even the year (RDR2, COD) with the playstation logo at the end of every commercial and probably even special bundles. I just mean that they'll have something to advertise so it's not like they will be going through the fall with nothing to show but commercials for games that have been out for months.
Strategically it makes total sense to push Days Gone into next year, hell I don't know if it was going to be ready this year or not anyway but with Spider-Man likely hitting in late Summer it's going to be a big year for exclusive releases for PS4, next year we don't know what's going to be ready. We know Ghosts of Tsushima, TLOU2 and Death Stranding are all being worked on but we don't know when they'll actually be released, it could very well be that two of those games don't make it until 2020 so pushing Days Gone to next year makes sense.