The fact that the trees we're talking about are easy to propagate and are popular does ensure their future existence, even if their habitat is degraded or damaged. This is indeed the case with the Dawn Redwood in China, where over-collecting of cones for its seeds is preventing its ability to propagate itself (its habitat has been degraded too). In an ideal world we'd fervently protect the habitats of rare species. I understand that this has happened in the case of the Aussie ravines which support the Woolemi pine, but it seems that this has not happened in those limited areas of China in which Dawn Redwoods are hanging on. Keeping species alive "in captivity" is second-best but it's better than letting species go extinct, or (as we do) force them towards extinction.