All of the speculations about the reasoning behind these changes seem a little off to me. They have not raised the bar (800k/400k permanent, or 40k/65k monthly). They have only attempted to make the proof of these numbers more legitimate. Seems entirely reasonable to me.
Personally, I suspect that Thailand has seen the potential coming tidal wave of expats. For example, in the US, the number of people who have saved sufficient money to retire is very small. A very large number of US citizens face a bleak retirement based on the only "retirement savings" they have - social security. However, take that 900USD per month from SS, and drop yourself into Thailand, and suddenly your standard of living has risen dramatically. Spread that potential across the breadth of the "baby boomer" generation, and Thailand could potentially see 10's of millions of people wanting to retire here. And, frankly, they simply cannot absorb that number. Nor, likely, wish to. Frankly, I see immigration requirements getting stricter over time due to this reason alone.