A lot of good questions, but 19 must mean you've never played a single MMO (or online game for that matter) in the history of your gaming life. Literally every patch update (not just expansion) makes almost all gear become 'less useful' with newer content, and stronger gear is added. The older gear works fine with all older content (just like the guns we have will continue to be fine with all the older Destiny content), while being less useful for newer, added content. This is normal progression for any online game that has any form of growth.