Your points are all invalidated because Destiny is a progression game, MMO or not.
Progression games always phase out old stuff in favor of new gear eventually.
You'll just have to learn to live with it.
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Where you are wrong is where MMOs phase out old gear, but destiny just replaces it. In a normal Mmo a new player can jump in and constantly get new weapons that get better as they do. In destiny, all legendary guns you get are the same attack value and will serve you all the way to the next expansion, and will be replaced again. New players can just grab a good gun right away and never get rid of it.