I don't think postgame is actually all that important to keep people playing for longer if what you have in the main story itself is of sufficient quality and quantity to satisfy. People often seem to place so much emphasis on what comes after the game that they neglect what is actually IN the game to be played and experienced. With Pokemon I can't say I blame them when the story is often so frustratingly threadbare and it's always the same core gameplay loop of beating gym, travel route, beat next gym, repeat until league, beat league, but wouldn't it be so much better if they spiced it up with things they could and probably would have saved for the postgame in the past? What if things like character side stories - think Buck and Marley from Platinum, and the whole Heatran sub-plot - were part of the main game, rather than postgame content, and added depth to the main narrative?
Personally I'd rather have a solid 30 hour experience (realistically speaking it probably won't last longer than that for the average player, and that might be too high an estimate really) with moments I'm going to remember and enjoy, rather than one where the main story is just something I have to wade through to get to things I actually want to do. There's no real reason for these things to be disconnected from the main narrative either, not even optional side events: they introduced the PWT in B2W2 pretty early on, as an example of something that isn't story related. Characters regularly appear throughout the game, and there's no reason their stories can't be integral to the plot and their character arcs resolved as a part of that plot so that their appearances have more meaning to them.
The whole idea that all the content people actually want to engage with needs to be relegated to the postgame is why Pokemon is often so unsatisfying to play through until that postgame, at least in my opinion. Given that some of my favourite RPGs don't really have postgame content and manage to stand as fantastic experiences in and of themselves without it speaks volumes to me about the quality of Pokemon games. I mean, there's nothing wrong with having postgame, and it is always nice to have something extra to do, but it's not essential if the core game itself is done right and done well, and that is personally what I would hope for before I start thinking about what lies beyond...and looking at Pokemon postgame historically, I see no reason why these elements couldn't and shouldn't be weaved into the narrative for a better experience when practically every other RPG under the sun would do this.