But..... does anyone worry that some of that might come back to bite them in the ass at some point in time when it comes to taxes / proof of ownership / other things.
Yes, is the never-ending story in this forum. It has been discussed in other threads, but not exactly from the point of view that you raise it now. o_e_l_e_o is one of those who advocates privacy to the maximum, I guess he will soon come by.
Lets say at the last big bullrun in 2017 I bought 10BTC @ $16000 when it hit $60000 this I wanted to sell and get a really nice house somewhere. If all BTC I accumulated back in 2017 was anonymous. I am going to really take it up the ass in taxes since I can't prove what I paid for it....since it was anonymous. So the IRS will take it as $0 cost.
It depends very much on each case. In that case, as I have argued in
other threads, the problem will not only be that they will apply the 0 cost, the problem is that you risk an investigation for money laundering at least. Imagine that you show up in front of the IRS with $600K in cash and tell them that it is from an investment that has worked out well for you and you want to declare it. But you can't provide proof of when you made the investment. You could end up like Al Capone for money laundering and tax evasion.
In addition, a 0 cost would not be so much of a problem if the benefit is 90% or more. In the case you put the price has only been multiplied by 4, but let's suppose someone got 10 Bitcoins at $1k and holded them until they are worth $50K, then sell. The purchase cost was $10K and the sale price $500K. In that case it makes no difference if the IRS applies a 0 cost to you than if they apply the $10K it cost you, the difference in tax is negligible. To me it's more problematic if you show up with $500K that you can't justify. That's assuming you're an average middle class worker with an annual income of $50K who let's say you have a half paid off house (with equity and you are still paying a mortgage), $70 in retirement funds and $3K in savings but also with some personal loan and credit card debt.
Showing up with $0.5M to declare in front of the IRS, whether in cash or Bitcoin is very suspicious.
In that case there are two possibilities. If you did not move the bitcoins in the first place, you can clearly prove that they are yours. But let's say you bought them on a decentralized exchange, or P2P from someone you knew, and for the sake of privacy you passed them through a couple of mixers over the years and there is no way to track them. In the latter case you run into a problem.