"investment returns" = "capital gains". Your marginal rate on "investment returns" in your country may be lower than in Thailand. If so, you would have to make up the difference.
I am talking theory. In reality, most people won't file.
You do know that the US taxes long term capital gains at 15%, right? Retirees often live off capital gains.
Unfortunately, Thailand taxes capital gains as ordinary income, which could be higher than 15%.
So for the large number of US Farangs in Thailand who receive income from capital gains, the DTA gives them a credit, but they still would likely have a Thai tax liability.
Most people here are discussing the letter of the law. But we are in a country where the police don't even enforce motorcycle helmet laws.
The real question is enforcement of Thai income tax, and I have no clue what happens when the million Farangs living in Thai villages (and who don't know anything about any of this) simply don't file.