I think that if they try to implement it, the Thai administration will have a lot of trouble managing the DTA issues, checking who owes what and/or managing claims and refund requests, enforcing payment on those who don't file anything, and ultimately, this is going to be an inextricable entanglement of problems which I anticipate will result in many offices simply not enforcing it because they won't even begin to understand and/or agree amongst each other what needs to be done and how.
I think you are correct in your assessment that increasing revenue is the key driver in this process: when the Thai authorities realise that this endeavour is having the opposite effect, i.e. people leaving in droves (and I know several who have already left), combined with a negative image compared to more welcoming neighbouring countries, I anticipate this law will join (or remain in) the long list of laws which are not applied.
I would not be surprised if they ended up charging a flat tax of 1,000 baht as a "processing fee" to every long-staying farang extending their visa, and all we would need to do is check a box stating "I certify being a resident of a country having a DTA with Thailand". If, say, 200,000 people are liable to the tax, that's 20 million extra per year for Thailand, just 1,000 baht extra per year for us, face is saved and everybody comes out satisfied. Now that, IMO, sounds a lot more Thai-realistic.