This is all true, but it assumes optimal data retention by retailers, and a government agency with the skills (and willpower) to do the legwork. In reality, I

This is all true, but it assumes optimal data retention by retailers, and a government agency with the skills (and willpower) to do the legwork. In reality, I would expect that one or more of those elements in the chain to be broken. Retailer lost their backups. Changed systems, threw out the old one. Doesn't keep records that far back. Etc. I agree there is a possibility they could track all these steps, but it would require everything working perfectly (from the authorities' POV)