The STT is a reason for market inefficiency because it pushes fast short-term traders out of the market. So inefficiencies tend to stay around in the market for longer time.
It defies common sense to think that STT does not affect in a negative way the volume of transactions. Especially with the tax as big as 0.25% on every transaction. And that I understand does not include the capital gains tax. How many traders you think left standing with such enormous tax?
The regimes in Iran, Venezuela are corrupt because of oil. Fortune that falls from the sky and that is not earned through hard work corrupts people and their mindsets.
You can't judge by just one outcome whether a strategy is profitable or not. My guess is that the strategy is profitable over long time even if the spread between pairs won't change.
People should not delude themselves that if they trade leveraged products that should be exempt somehow from transaction tax.
How about spread traders? Should they be exempt as well? I don't think those trading nuances are on regulators' mind.
In this case no wonder the OP thought that this is a risk-free trade since the PnL chart shown is not taken at expiration but on some who knows what day.