You are correct here. If this policy were to be actually made into a law, the baseline rate of return would be hotly debated, and would need to be defined in relation to some sort of metric.
It would likely end up being the retrospective 10-year T-bill rate, or some other minimal-risk rate of return (although have multiple independent sources of data that are averaged would be needed to avoid manipulation of the metric—eg if you just take the 10-year T-bill rate, all the big money can avoid that investment, thereby driving up the rate and giving them an huge tax advantage on their bitcoins or foreign bonds or wherever the money actually went)
You are correct here. If this policy were to be actually made into a law, the baseline rate of return would be hotly debated, and would need to be defined in relation to some sort of metric.
It would likely end up being the retrospective 10-year T-bill rate, or some other minimal-risk rate of return (although have multiple independent sources of data that are averaged would be needed to avoid manipulation of the metric—eg if you just take the 10-year T-bill rate, all the big money can avoid that investment, thereby driving up the rate and giving them an huge tax advantage on their bitcoins or foreign bonds or wherever the money actually went)