Have you worked on a product that has had to show a vertical slice 2 years before the product is released? I've worked on a few e3/e3-like demos for games and there's a whole bunch of issues. It's not always quality - things that were considered "easy" or "we can solve that later" can come back and bite you. Things that are fun in a vertical slice demo might not actually be engaging for more than 15 minutes or so(or an hour, or three). It can be _very_ difficult to extract a 10 minute piece of gameplay that you have some context for to show people even if you have the entire game made, which means you might have to out in some temporary "features" (add a power/remove a pathway) to make the experience fit. All or these changes can have a butterfly effect. Adding X to simplify a demo can make Y seem more important than you want it to which means Z is now redundant, but you've already spoken about Z.

There had to be done realisation that things change in games over a 2 year period, (heck, things change massively in games over a 2 week period in some games) and some acceptance of that fact.