The Descendants were old and rusty even in D1, you’re thinking of the Precursors which will still have the sleek white and gold look in D2. Bungie doesn’t seemed to have changed the look of these two groups much like they did with the more present-time Vex models The Fallen have less armor because (if u pay attention to enemy titles, a few dialogue snippets, and a couple scannables) they Fallen are actually extremely weak and low on resources now. They all banded together but are scraping the bottom of the barrel and devoting all resources to recovering old supplies or stealing new supplies. They don’t have enough armor to go around so they’re a lot like how the Exiles were in D1 (threadbare cloth, little armor, struggling for a foothold, undernourished, barely surviving) I believe that one Vex from Venus (the gold one in the tree where Xûr appeared on Nessus) actually had the old D1 look, so maybe it’s supposed to be like only these Vex from Nessus and Io have the new models? There really isn’t an explanation I know, so it’s probably just Bungie reskinning. But I guess if u want to just come up with an answer we can say the Vex either adapted their models or some from a different time popped up (the Vex can probably switch out older forces for newer forces in a short amount of time due to their ease with time travel)