The big problem Anthropic faces isn't implementing a KYC workflow, but the fact that many if not most of their own employees are no longer allowed to work on Fable/Mythos.
What a hilarious situation. Clearly, they never even imagined their precious government regulation would be turned against them.
Also their API customers and downstream customers (e.g. Cursor users) would also need similar infra, and probably a decent amount of users would just choose another model that doesn't require ID & an immigration status check.
And API is much more profitable (relatively) than subscribers for them.