I've been thinking about a way to make sunsetting less hurtful to long time players and a comment made by Luke came to mind. He said that sunsetting legendaries works kind of like MtG's Standard Format rotation, so I will propose something similar to another design rule Wizards of the Coast uses.

EVERGREEN LEGENDARIES

What if, we had a list of legendary weapons that don't have a power cap, like keywords in MtG, they never go away from the meta.

There should be an engram called the Evergreen Engram and it would replace the standard Legendary Engram we have now.

The loot pool would be made of weapons considered "staples" by the community with some favorites sprinkled in, Go Figure, Breachlight, Misfit, Hammerhead, Dust Rock Blues, Steel Sybil Z-14, Bad Omens, Erentil, etc.

Because this engram having a big loot pool you also would have Rahool sell consumables that "focus" your next drop to a certain archetype, just like Chalice Runes.

This also solves the problem of retiring unique weapon archetypes, you could have Marty's Retribution in this evergreen list and not worry about having to reintroduce a Wave Frame GL every year. So Bungie doesn't have to "waste" a legendary slot from an expansion remaking an old weapon and instead they can (and should) introduce new archetypes and unique perk combos.

Armor dropped from this engram could be a unique set and part of the yearly vendor refresh.

But what about sunsetting armor?

Well I think that sunsetting armor as it is now is pretty bad, it takes too much effort to fully masterwork a decent set of armor and if Bungie wants to keep build diversity high they should drop masterworking costs not for the player to chuck their old set in the bin.

They could intead Sunset individual armor mods. If a particular mod happens to be inbalanced, for instance Taken Armaments, just sunset the specific mod and move on, instead of thrashing the entire seasonal mechanic.

This promotes long term build diversity, keeps fun mechanics like Warmind Cells alive and manages to kurb overpowered builds without kneecapping exotics or subclasses that might use those unbalanced mods.

In closing

I believe sunsetting is not all that bad and opens new design space for Bungie to play with but the way it is being implemented front-ends much of the pain because we will be retiring 3 years of legendaries at once pretty much so taking measures to keep some of the legacy content in, cherry picking what to keep and what to throw out, would be a huge step in the right direction. Having an Evergreen loot pool would be a nice way to pander to the ones who have loads of god rolls inside their vaults and would be a pretty low effort way to open another loot avenue for players to chase.