Has anyone ever been convicted for doing a few on-exchange trades and conversions? You should be keeping your receipts for tax purposes, it's on you to provide them up to a statue of limitations.
Or is this just a theoretical concern for anyone who isn't laundering Bitcoin stolen through ransomware or from exchanges?
You missed the point. The point was that there are many ordinary and mundane reasons why you wouldn't be able to prove the full chain of custody for the BTC you acquired in 2012. It would be a real practical concern if you are a law-abiding person. If you're a criminal, it's not much of a concern since laundering money is relatively easy.
Yes but the claim is those are hard to track details, and then what's described is a timeline of consistent interaction with the money which you're now planning to claim you have no knowledge of.
And some of those interactions are taxable events - e.g. if you are exchanging out of cryptocurrency denominations, then by US tax law as a US citizen that was a taxable event.
Chris Borden went to prison for Bitcoin related financial malfeasance.
https://youtu.be/cuIRvn89988