Let's say you got bitcoin (or any other coin) back when it was very cheap. You bought a hardware wallet, and you held it through all of these years. It's now worth hundreds of thousands (or more). Is it safe to say that you're still likely to be asked for the source of these funds even when they can see the funds has been moved from a dormant address?
This is simple. The government/regulators are on the neck of any centralised crypto business you are dealing with, or the banks that will be created in fiat. And the larger it is, the more they would want to make you accountable for it.
The proof of the source of funds is for the AML check and to fight terrorism, and I think your response to that should be easy as well if the initial source of the funds is clean. You can simply prove the source of the money you invested when Bitcoin was cheap, and the prevailing Bitcoin market price will prove the remaining source.