If they spin it off, it likely wouldn't IPO, Amazon would be the only stock holder. All profits from AWS are already factored into Amazon's stock price, another IPO would be unnecessary
Amazon Web Services, Inc. is a separate corporation wholly owned by Amazon.com Inc
I agree, it seems unlikely in the near future given Amazon's history, but it could happen. Maybe 10-20 years from now when Bezos has retired, a new CEO or three has been in place and the public markets aren't as forgiving of Amazon as they are now.