The difference with previous shifts, even if they were as rapid as the one we're now speculating about, is that humans have fragmented the habitats of (at least the higher) species to an extreme degree. Migration is going to be a lot more difficult. On top of that we get the extinctions already caused by direct human influence and invasive species spread by humans. So we have a very dangerous situation.

I'm not an ecologist, but my guess would be that most of the top of the food pyramid comes tumbling off if we continue as we do, and a lot of ecosystems will go into disequilibrium, causing further extinctions. It's a very dangerous situation for the human species to be in. We're not technologically ready to survive as more than a rump if the ecosystem services we rely on fall down. The amount of social upheaval that will accompany this transition makes planning it impossible. I'm pessimistic about human survival in the face of catastrophic climate change.

You seem to have a very casual approach there. Life would take the mother of all blows from a rapid melting. The continents' shores are polluted enough that it would wipe off most of the shallow water marine life. Species are NOT trained to such a rapid change.
A lot of species died in the climate change periods of the holocen, and they were not nearly as rapid as what is projected. But also, species had a much larger and undamaged habitat then. And the shores were clean. Finally, there was no dominant species to kill all animals bigger than a grasshoper because it was starving from the huge reduction of its agricultural land.

It would be an unfathomable catastrophe.

Now, I don't expect +70m anytime soon, and by that I mean within the next million years. It would be too catastrophic not to take the most drastic of actions.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi