Given that nightfalls and PoE now only drop TTK-tier loot, you're not missing out on those as loot sources for Y1 gear. So you can still play Y1 content and get your Y1 gear just fine. I agree with the invalidation of the previous endgame. It'd have made much more sense if they just removed XP levels and switched over to the new level entirely, or if they kept L20 cap and did the same. As the old Raids don't even give XP, or have quests associated with them, it's sort of pointless.