There's no reason you would be a criminal, if they're comfortable taking so much cash (you didn't specify how much, dealers may vary in how willing they are to accept very large cash purchases) they will doubtless be comfortable filling out the mandatory reporting.
The investigators are looking for patterns, and latchkey having one dead grandma and buying one car isn't a pattern. They can't see patterns if the data which would make up the pattern isn't recorded, so that why the data is collected.
Or even just index it to inflation. $10k in 1970 is $80k today. One of those could be first & last month's rent in a major city or functional 3rd hand car, the other is at least adjacent to an interesting cash transaction.
Car dealers' comfort is proportional to governmental scrutiny and control of their finances and transactions, so I think this is still a big deal as we move further and further away from the ability to independently transact among ourselves without controlling middlemen (government, banks, etc.) centralizing and controlling this ability. Not to mention the big banks and governments moving to blockchain financial vehicles they sanction and ultimately control. Soon you will have to be fingerprinted for a SIM card here like in Saudi Arabia. I've lived in SE Asia where you can buy a SIM card from a convenience store. To get one in KSA, I would have had to have had a government ID or number, and they fingerprint you to get an account. You know, because only criminals, drug dealers, and others use "burner" phones; it's not about trying to track and control you ;)
The investigators are looking for patterns, and latchkey having one dead grandma and buying one car isn't a pattern. They can't see patterns if the data which would make up the pattern isn't recorded, so that why the data is collected.