AI-based Consensus Mechanism - over time more and more parts of the validation process can be automated/powered by AI. Though initially these will just assist validators, at some stage AI can be directly integrated into the protocol until AI replaces manual reviewers entirely (the AI will be trained on the inputs from validators until it is sufficiently effective at replacing them).
No user will be expected to validate the entire AI process - that is of course impossible. The process will work based on Validation by Sampling, where the entire AI response is chunked into verifiable subsets, and each subset can be verified by users.
Since a verified process doesn't ensure that the AI's output is exactly the project's economic impact, only that the system correctly produced a result based on the input and training it received, the user-based challenge period will remain.