The LINK token is required to be staked as collateral in LINK nodes.
This collateral must meet or exceed the cumulative value of the smart contracts that it supplies data to in order to establish a trustless environment as defined in the Chainlink whitepaper.
These smart contract values are things like Aave (5.2B TVL), SNX (2.5B TVL), and of course the hundreds of other projects that will rely on Chainlink data feeds.
Therefore, the marketcap of LINK must meet or exceed the cumulative value of all smart contracts which rely on it for data.
We're talking hundreds of billions to trillions, assuming smart contracts continue gaining traction and crypto doesn't collapse entirely.