I don't think we're talking about the same thing at all. What happened in the Forerunner Trilogy happened, but not everything in the past is that important with what is happening "now". It's clear that a lot of the Forerunner Trilogy *does* matter—seems likely we're going to get a return of the Flood, the Domain and the ideology and dogma of the Mantle, possibly some of the locations previously seen. If it were akin to a Bungie book/game split the most we'd get is a passing reference in the game to a quote from the books :)

On a side not, I'd honestly be surprised if the Librarian persisted as a major character or force in the lore. We did get 343GS's damaged essence certain he knew where the Lifeshaper was, but for what we know that could mean Chant-to-Green as opposed to the Librarian (I think from a story standpoint it's also better if she didn't survive, because otherwise the sacrifice in Silentium/the H3 terminals is a bit more hollow.) She tried to guide humanity to reclaiming all of the Forerunner's legacy, and humanity got smacked down for their trouble (The Absolute Record). In that sense, her big plan has already failed, at least directly.