The IRS is not limited to the technical realities of Bitcoin in coming up with explanations for how it interacts with the tax laws of the United States. "Individual coins cannot be traced on the blockchain" doesn't affect their operations in an iota. The IRS is well-adapted to environments where there does not exist a single public record of every transaction, principally because that is pretty novel.

If the IRS believes that people are misreporting bases in such a manner as to meaningfully affect their tax obligations via using Coinbase, all they have to do is send Coinbase a letter.