Interstellar arks are all well and good, but we still need to have a complex infrastructure in place in the Solar system in order to construct just one such vessel, which would be a massive project. Not only that, but we would need
considerable practical experience in constructing and maintaining self-sufficient spaceborne colonies of appreciable mass - we'er talking something along the lines of
Island Three here.
I don't think 8 million tons is enough for an ark. It might be good enough for a
sleeper ship or
seed ship, or for a ship with a biologically immortal/cyborg/robotic/digital crew.