It’s to keep us grinding for armor and upgrade materials to master work it. If you found your god roll armor in season of the undying last year, and master worked it, you won’t grind for new armor now. Honestly, when it comes to armor I’m not a fan of the sunsetting. It’s too much of a pain the butt to get the roll you want due to how many stats there are that gets points allocated. It would be a bit more work, but I’d say having armor with set bonuses, or intrinsic perks would make people want to keep grinding without Sunsetting. They kind of do this with the Raid armor being able to use Raid mods, but yeah, things kind of like that. Or like Dreaming City’s Riven’s Bane perk, that I don’t think even works any more. Maybe raid gear has the mod slots, so you can get a benefit with just a single piece, but non-raid armor sets have intrinsic perks that only activate with 4 and 5 pieces of the set. To encourage changing down a full set and then chase good rolls of that set. The set bonuses can be location or activity specific, so they can be more tailored and you could do something like a boost to super generation in non-GM strikes, but it won’t work in Crucible. If the bonuses are interesting (fun or generally beneficial) people will grind for those sets and then want good rolls. It doesn’t even have to be a static thing. Maybe the vanguard set bonus gives you an additional random beneficial daily modifier. So like one day you also got more power weapon ammo and damage, the next it changed to a singe other than the weekly singe, the next day something else. It doesn’t have to lead to power creep, because set bonuses can have restrictions and exclusions. Maybe set bonuses don’t work in GM or Raids, unless they make a set you get from GMs and Raids tailored for those activities. Some bonuses may only work in some activities, or some locations, in addition to having exclusions. Introducing set bonuses for armor sets would give us more reason to grind than sunsetting armor, because unless your armor just sunset or is just about to sunset, there really isn’t a drive to grind for another similarly rolled piece of armor to replace the one you have.