you say, "I get that it takes a lot of time to create new content..." like that means anything. 99.7% of new weapons and armor in Destiny have always been reused assets with new skins. Look at the new Vanguard Pinnacle weapon, reskinned Hung Jury from D1. Hung Jury was a reskin of something else. Instead they spend time making this fugly ass ghosts, sparrows and ships. Don't get me wrong, it's nice having some options but those have ballooned to near ridiculousness. You are correct though. One issue with D1 was that often Bungie made our old gear/weapons irrelevant by lowering their power value and forcing us to acquire all new weaponry/armor. They have addressed this issue and now allow us to keep the items we like, well, kind of anyway since removing mods from D2 Y1 made old armor irrelevant. But new weapons and armor are a necessity. I'd prefer Destiny be an MMO but it's clear it's more of a so called Looter Shooter. Regardless there needs to fresh new shiny toys to chase. Granted there's going to be some weapons/armor of old we prefer due to look or function but there needs to a continuous flow of new content including armor and weapons. Though this is one of many issues plaguing Destiny. I will agree and say that it's one that should be easy to fix. re-skin some D1-D2 weapons, modify the stats slightly, perks, whatever, easy peasy. Maybe even throw a few new items in for good measure.
The fact that there are more new ghosts, ships, sparrows, etc than gear each DLC is definitely a problem too. I get your point on the redone weapons too. Have no answer there. It would seem we'd get to a point where they'd start really adding new weapon and armor designs. In D1 there were pretty distinctly new designs in most of the content releases.