I think if a loophole exists for a collectible asset class wealth will accumulate there as part of tax planning. This can easily lead to bubbles and unfairness through asymmetric tax avoidance. I think it does pose deep foundational challenges for a wealth tax. Tracking income is far easier and many of the most common abuses defer tax rather than permanently avoid it. Under reporting of a hard to track quantity would result in less taxes permanently in the new system and measures to avoid it would represent a new level of reporting to government that many would find deeply invasive. Given all these flaws are the benefits large enough?