I took a bit of umbrage at the idea that humans surviving longer than, say, the dinosaurs did is "over hyped and misunderstood". That comment was my attempt at an unhyped and easily understood definition of "sustainability".
Three hundred mega-years (what I meant) is a somewhat arbitrary milestone. (I think long-term but 10^15 years is a bit much for even me. But see Stapledon's "Star Maker"...)
(BTW, your thinking seems unusually clear on these matters.)