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Looks like you can't renew your passport online if it expires in more than a year. The form makes you verify this statement:
The passport I am renewing is either expiring within one year or has expired less than five years ago.
But I did call and find out you can still renew by mail. (I also tried using travel.state.gov's online chat, but that agent, whether LLM or person, was worse at searching their site than I was.)