I still say the severing really wasn't necessary. It seemed like it was done to just to raise some not very delicious steaks.
I liked it. It fit with the themes of Book 2, and made the battle with Unalaq and Vaatu feel like it had lasting consequences. The scene with Unalaq destroying Raava and all the Avatars vanishing is one of the the most heartwrenching moments in ATLA/LOK for me; going back on it would have cheapened that scene.
Well, I like to assume that the old Avatars are still available in the Spirit World, but the quick-line to them was severed, so one must actually look them up to get advice.
Nah, I prefer to think the opposite. The past Avatars are gone, period. If they're still accessible somewhere, that makes the ending of Book 2 less meaningful. And it makes Korra look like an idiot for not going in search of them at any point.
The events of Book 2 aren't needed for Korra to look like an idiot. It comes naturally to her.
Like how Aang searched for the past lives while he was dead and recovering thanks to Katara between Earth and Fire, AtoMaki's assumption isn't too farfetched considering we see the line of them disappear and the glowing Aang cosmic representation fade and fall.
The lasting consequences also only apply to the future Avatars after Korra.
Aang was fine because he couldn't figure out the ghosts, but Korra had Aang showing up and solving all of her problems without asking; that had to go if Book Balance was going to happen.
Roku showed up at times without Aang's prompting though. And Book Balance didn't need to eliminate the past lives because Raava (Aang if he was still alive) could have told Korra "I'm always here" like Raava did once her would-be murderer coached her because Korra never learned anything herself through her own merit and applications.
The only thing that was needed for Book Balance to happen is for Korra to forget that she is the Avatar on the mountain peaks before being poisoned. Since she forgot she was the Avatar and could defeat Zaheer with her skills, Book Balance happened.
It's a weird thing though. Raava still holds the elements despite losing her connection to Aang going back to Wan. How? If she can no longer make Aang going back to Wan appear, how can she still have Korra bend the elements despite dying and being reborn in Vaatu? If the soul of Wan is Korra's, then that's her own memories/experiences through the lens of Raava. Raava shouldn't be the one holding the memories/experiences. Reincarnation means the person doesn't recall their past life. That's what made the Avatar of this universe unique. They could remember and call up who they were before in another life.