Rising isn't supposed to be "some crappy spinoff" by any means. This whole backlash because it's a multi-platform game is about as ridiculous as brand favoritism.
The caveat is that Rising is being developed by a new team without the guidance of Kojima which may well change the end result if they're really shooting for something that distinguishes it from previous MGS games.
I think the reason why Kojima is considering Peace Walker to be the sequel MGS5 is simply because it covers a pretty vital portion of the the MGS canon, rather than being a side story, in addition to it being another Kojima led project that he's closely involved with.
It is kind of unfortunate that a lot of people will be missing out on playing this who have little or no interest in the PSP or portable gaming. It will also mean a lot of potential lost sales, but it's no entirely surprising considering that there have been 5 exclusive MG related releases to date for the PSP, even if one was a digital comic and two were strategy turn based games. Kojima must like the PSP for whatever reason.
Personally, I'd rather play Peace Walker on the PS3, with the one big plus being a faster development time on the PSP. I'd even prefer playing the same PSP game through the PS3 on a TV rather than a little hand held screen. I just don't see it as a game for playing on the go in 5-15 minute sessions.