First, I'm not saying the rumor is true (though it wouldn't surprise me).
The problem is that the 3DS successor is nowhere to be seen (the system was released early 2011, so it has been around for 6 years). The WiiU factors in, because console fans of MH were abandoned. Sure, the games weren't that huge sellers on consoles anyway, but MH3 sold close to 2 millions on the Wii, that's still money left on the table, also they need to update those assets/code/engine or whatever for modern hardware, and with 3DS it wasn't possible. There's also the fact that the 3DS isn't setting the charts on fire in the west, although the MH games have sold very well.
Seeing how secretly Nintendo handled the Switch, and the quality of third party games released for it at launch, even the quality of first party games (port of Zelda, port of MK and 1-2 Switch), I would say third parties new the details less than a year from launch, honestly. If they had let third parties know before, leaks would have happened sooner.
Switch is the 3DS' successor and always was.
Nintendo talked to third parties about NX at E3 2015 and Capcom is literally their closest partner so they probably knew about it before then.(Actually I'm fairly John Harker said they were sharing details before then and he's good peeps).
I don't remember that but I'm talking about Nintendo locking down MH to the 3ds.
Why would a September real at TGS make more sense for a supposedly western oriented MH5 on the PS4? E3 is the most likely venue for this particular rumor.
Because you don't announce a game for August and then supplant it with another game, especially considering according to the rumour Sony wants a short time between announcement and release.
EDIT:
Simply because its potentially a new game in a series and nothing is known besides a rumour with questionable legitimacy. It isn't like there is a DmC style reveal for people to react negatively against and so far the only element people are whining about is a rumour stating that it isn't on their console of choice. What Capcom could do might not be to all fans tastes but the possible options on what they could do may appeal to many of the fans. There isn't enough to say one way or the other and a rumour based on what system it is on doesn't really say much about the quality of the end product.
You know except the big part of the rumour detailing gameplay changes and talking about how there's a lot of conflict within Capcom about it.