The "debit card for APIs" framing is exactly right. One thing that doesn't get mentioned in these x402 discussions: every one of those micropayments is a potential sales tax event. The moment x402 scales to the point where a developer's agent is making thousands of paid requests across state lines, they've likely crossed economic nexus thresholds in multiple states without realizing it.
The irony is that x402 solves the API key problem beautifully, but the compliance overhead it creates is invisible until it isn't. Sales tax law doesn't have an exemption for novel payment protocols.