I'm not sure I agree. This only doesn't disincentive productive activity if you think that secondary financial markets (including speculation and market making

I'm not sure I agree. This only doesn't disincentive productive activity if you think that secondary financial markets (including speculation and market making on those markets) aren't contributing anything productive. I'm of two minds about this but I lean towards the notion that speculation is a net value add. Think about the trillions raised in the secondary market, which is all contingent on valuations set by supply and demand, where these valuations are determined by speculators. The more accurate the speculators are, the more productive the capital is that's injected into the secondary market. So I think we definitely don't want to disincentive the institutional speculation that happens, which is what a tax would do. On the other hand we want to disincentive retail hype bubbles like GME, which lead to the opposite to the above - capital going the wrong way. However it's not clear to me that the tax would achieve that. Retail hype seems rather price inelastic. The bitcoin craze in late 2017 was good evidence of this, since fees for trading crypto are very high.