Possibility of nothing but all-encompassing, absolute Nibbana without Samsara? If so, is sustainability possible (and how)? Is nothing but all-encompassing, absolute Nibbana without Samsara possible? If so, is sustainability possible (and how)?
I think that nibbana is defined as the end of suffering -- which occurs with the end of craving (as defined by the noble truths).

The end of craving happens (permanently) when the various causes of craving up uprooted -- the causes of craving include e.g. anusaya and asava and so on.

This uprooting can be permanent in the same way that when a palm tree is cut then it won't grow again.

Conversely, Samsara is a cycle of existence -- the pratītyasamutpāda doctrine (i.e. the doctrine of the 12 nidanas) says that cycle (including further birth) is conditioned by craving -- and so does this description:


  “Mendicants, transmigration has no known beginning. No first point is found of sentient beings roaming and transmigrating, hindered by ignorance and fettered by craving.