This is what I hope they do with the next game-
Make the game with all text. Once the game is "finished" then hire the voice actors. That way, all the changes have been made, all the characters are fleshed out, the writers are happy with how they matured and developed the dialog, and the actors read their lines then the game is released.
Oh, but that would be sensible!
Committing to fully-voiced NPCs simply creates a 'tail wagging the dog' situation. NPC interactions should be limited by the writers' imaginations not the amount room left on the disc for sound files.
Can't really fault Skyrim for lack of voice actors. Seventy should be plenty. Still, I can't help feeling the results should have been better. I can't put my finger on whether it's the actors, the writers, the sound editors or someone else that are most to blame.