I think that might be a huge reason of why they want to sunset stuff. Like you said, most of that content will be completely useless, so places like the leviathan might just go away. They might be using this as a way of faxing out old stuff to make room for new stuff, basically getting rid of stuff that takes up a bunch of space allowing for new larger content drops. But, I don’t know, I don’t work at Bungie.

They will be. Look at the past, in D1 Y1 when ever bungie released a new dlc they "sunset" all the old gear and while yes you could use it in crucible or patrols . The meaningful content always requires the current light level. So since sunsetting is supposed to start in s11, that means everything pre opulence will be under powered. This means forges, menagerie, reckoning will be pointless. Also if you notice gnawing hunger will be in the world loot pool for s11. The only reason why bungie would take gear from those old activities and add it to the world pool, would be because they are considering removing the original source. Also removing Titan, like some people think will happen, would ruin the campaign for new light players and remove a strike, plus 2 exotic quests. So Titan is more valuable than say old seasonal content, which has no real effect on the overall story. Not too mention they're removing the seasonal content for Y3, and moving away from seasonal activities as sources for seasonal loot.