None of the old things you mention would be around if they were not continuously reinterpreted for the "modern audience.
That is not why.
It is the classic and enduring nature of the work that makes people want to do all the alternate 'interpretations' that merely come and go.
It's the original Shakespeare that you can find in bookstores, and that is still is widely performed in plays that keeps captivating people's imagination.
The same with Tolkien. Yes, we will get animated this, movie that, and bad tv series whatever.
But it is his original works that are still widely read and captivates people's minds that makes them want to adapt it to other media.
if these things are changing in a way that upsets you or makes you feel left out, that is the system working as intended. If it is going to survive, it can't be solely for you and your cohort anymore.
The reason why the original works of Tolkien and Shakespeare will continue to survive beyond any given adaptation is because they touch on universal themes that are cross generational.
Shakespeare has been around for centuries, and Tolkien is pushing 70 years (2-3 generations), and it is their original works that are readily available in bookstores worldwide.
I wonder if Shakespeare and Tolkien have withstood the test of time in part because they are finite things.
Continuity does help...
Is D&D kind of like these? How much would it sell if it was Gary putting monthly updates of Greyhawk into Dragon Magazine?
It would sell well at first, but then it would bloat and wither away.
We have seen this effect in games that did a lot of metaplot for their settings. They can start out well, but eventually have to end. And the people invested in the setting have nothing to follow up on once it is all over.
Which is why in my opinion; after the set up is given in a guide or folio, RPG settings should be static.
Each group should be encouraged to make the games campaign their own during play.