Why on earth would the original attachment theorists have suggested that all that attachment work gets done in the first year or two of life? As if later experiences don't mediate those early patterns of behavior?

Later experiences may mediate those early patterns of behavior, but initial experiences hard wire the brain into certain set patterns. Those set patterns can be altered through normal experience, but it takes a whole lot of experiences to erase that initial imprint.

Actually, that's not entirely true. There are ways to wipe old imprints clean so that you can re-imprint, but they're rare. Most people have to muddle through life with an underlying personality structure that has already been defined by a handful of early, high-impact experiences.