I think you need to have a good hard think. Almost every retiree I know in Thailand uses an agent to facilitate their extension of stay. Not one of them has 800,000 in a Thai bank. I'm guessing the number of expats using an agent runs into the thousands.
Do you think immigration will want to lose this source of income? No they won't, in fact if and that's a massive if these tax proposals ever become law, immigration and the agents will find a way around it, for a fee of course.