There is a physical limit to the standard chip size. Once that is reached or breached we should see some leveling.
Also a lot of the market is driven by consumers and large mining farms.
Consumers are driving up the price as non-knowledgeable peeps pay huge premiums for miners.
Large corporate mines are buying up massive hashing power with massive non-disclosures and financial backing.
I think individual mining will see a slow fall off to cloud and corporate mining just like no one ever intended.
People have build transistors out of a single phosphorus atom. And Graphine transistors could potentially use far, far less power then silicon ones. There's a long way to go, in terms of absolute scaling before we hit a wall, although on an exponential axis it's probably not that much farther (so we can only 'double every 18 months' for so much longer)