Thatching material is harvested by removing selected branches, or fronds, from a tree, leaving the tree and other fronds to grow.....not by cutting down and thus killing the tree (although I have heard there are some who harvest thatching in just such an incredibly short-viewed manner).
The longevity and low maintenance of this type of roofing surpasses 'zinc' roofing, particularly close to the sea (which we most definitely are) where salt in the constant breeze takes an immediate toll on everything. Maintaining a metal roof can be a full time job. In fact, watching 'zinc' rust is a time-honored pastime in the Caribbean.
And I've been told that when a falling coconut hits a metal roof, at the very least you've got a dent in your roof. Those heavy suckers bounce and roll when they hit thatching.