LOTRO really doesn't have a choice. Once they finished the books, where else could they go but to new lands?
It wouldn't be that difficult to put new content in places like the Tower Hills, Lindon, or other vast swathes of Eriador that have yet to be covered, where they wouldn't need to create new assets for the terrain, etc. Lotro doing the equivalent would be a return to Moria, where we go down and fight a balrog who also happens to be in the deeps, but was, I dunno, more chill when Gandalf showed up unannounced on his doorstep.
Even then, a Lotro expansion where we venture into the depths and face the nameless things that gnaw at the roots of the world as Gandalf described it, would have me more interested. To be fair, though, I have never been a big Underdark, Arcane Trickster, Duergar or Demogorgon fan to begin with. It could simply be they picked the exact things that don't interest me, and I am alone in the disappointment.
If they had announced Sigil, added the elemental Genasi races to round out the core planetouched, and said the adventure was based on the Modron March culminating in a fight against Primus/Orcus reborn, and they would be introducing the Swordsage archetype for fighters (Or a Tome of Battle/weaboo fightin' magic universal tree), I'd be a lot more excited.