"5nm is far too close" ... for ... what exactly? What is it about 5nm that makes it "too close"? What occurs, or stops occurring at 5nm?
Is this a trick question? Casual checking in...
At some point down there you get too much electrical leakage to carry a signal.
Classical mechanics break down as quantum effects begin to appear.
Registers begin to act too much like qubits for digital reliability, which requires static discrete states (1 or 0).
The easiest gains were at the beginning. Moore was observing the harvest of the low hanging fruit, nothing more.
Now it takes more capital to build a modern fab than it took to go to the moon.