See... the thing is: All the old lore is still there.
And just like it has always been, it's there to pick and choose.
You wanna run a 5e game in the Time of Troubles? Go right ahead. No one's gonna stop you. Update Ruins of Myth Drannor for your home game while you're at it.
They certainly haven't ADDED a boatload to the body of lore that D&D has across it's various settings. And the changes made (Ravenloft for example) haven't always been met with resounding cheers... but it doesn't really matter.
Because it's all out there. Every book still exists. And WotC sells more and more of them every day on the DMs Guild and stuff.
You want the Dark Sun Boxed Set? Here y'go.
Dark Sun Boxed Set (2e) - Amid the barren wastelands of Athas lie the scattered city-states, each in the grip of its own, tyrannical sorcerer-king
www.dmsguild.com
PDF or soft-cover. Your choice. Shipped or downloaded however you like it. It's still there.
Does that mean they're not going to make more content for old settings? CLEARLY that's not the case as Ravenloft and Spelljammer and Forgotten Realms and more continue to trundle down the pipeline to us.
But it's a trimmed down version great for introducing new players and DMs to the setting, with old soldiers like ourselves able to say "You're wetting your toes, but if you look here you can swim." and hand them the old books.
They're just not -glutting- the market like they used to.