Yes, it is the wealthy or super wealthy Thais who will be most affected, as they also currently enjoy the tax exemption for worldwide income, repatriated in a year subsequent to the earned year. I would expect, as you say, they will be the ones who press the issue and if they don't succeed the monies wont be repatriated, which will be a negative for Revenue tax gains and economic activity.
Thai resident, super wealthy foreigners, who are a much rarer species, probably can protect their assets and depending on their visa, may even still be exempt. And if all else fails, due to their resources, they can relocate to greener pastures.
Resident foreigners, bringing in income, live, were always, if resident more than 180 days in a tax year, technically taxable here, a fact, I suspect, which went largely under everyone's radar, including the authorities. However, if their income derives and is taxable in a DTT jurisdiction, it is very likely that the tax deducted would be greater than Thai tax liability, so, therefore, no change.
The Thai income tax is gradually graded from 5 towards 35%, with attractive exemptions. For instance, a person, who is over 65, married with one child, has an allowance of 460,000. That person, say with an income of 1,000,000, because of the grading bands, would have a liability of, maybe, 60 or 70,000 baht, which would be, most likely, below taxes levied on similar income in most Western countries.