What would happen if all-time classics were written now?

Hamlet is not a contemporary story - the OP does not contemplate rewriting the classics for our time merely that they never existed before.

The Lion in Winter is a latter 20th century work about Henry II. It is praised for its dialogue, which was not written in contemporary style.

So too the newly issued Hamletwould be considered a period piece.

The Breakfast Club would be a lot different if it was made today.

Of course there have been transcendent authors in the last 100 years. Nabokov, to name just one.

Then again, I don’t agree that the fall from Tolstoy to Joyce is nearly as immense as you claim.