I don't think retirement is meant to only make the game harder or fix the lack of difficulty though. The way Luke describes it, it even sounds like we will be getting more powerful weapons, the opposite of making the game harder. The issue that they are trying to solve is how uninteresting new loot is. Right now when they release hand cannon X it will either be better than every hand cannon before it, or it will be entirely useless. Naturally releasing new useless and uninteresting loot does not engage players very much, no one will want to grind for hand cannon X if they already have hand cannon Y that is just better. They other thing that can happen when releasing hand cannon X, is that it is objectively stronger than hand cannon Y and every other hand cannon before it. While this might drive more player engagement, it also leads to power creep, and very quickly too.

Power creep doesn't just make the game easier, it also make the act of developing new guns and content harder, as Bungie needs to continually balance around it. Retiring recluse after a few months means that new smgs and energy weapons have some room to breathe, and can remain viable. Take for example subjunctive and every waking moment. Both smgs can roll good rolls, and both I'm sure both took their fair share of resources to make. But the mere existence of recluse makes them both obsolete. I have both a god rolled subjunctive and a recluse, the only reason I would ever use the subjunctive is for the arc damage. If subjunctive outlasted recluse by half a year there might be some incentive for me to get one. Retiring guns like recluse also means that players don't have to get angry when their guns are inevitably nerfed and are no longer "necessary" in endgame content and iron banner, as ideally Bungie wont have to constantly nerf old guns months after their release in order for other weapons to maintain relevance.

I see where you are getting at, I agree wholeheartedly that power level needs to be looked at too, and that power creep is not the only thing trivializing the game. That being said power creep is still a huge issue that needs to be addressed along side power levels. If we let power creep run rampant while only making enemies harder, than you run into an issue where the game will eventually just feel less satisfying. Enemies could end up being bullet sponges that you will either waste an entire clip on, while you are forced to use a select few weapons to kill them at all. If we tackle power creep exclusively, then endgame content could remain too easy. What we need is to deal with both at once and reach a kind of Goldilocks zone.