1/ If you have less than 65K income and money invested that gives a variable income paid quarterly this is not an option unless the old system is still working where you can have less investment the nearer to 65k you can get so you need less in a "dead" bank account. Say you get 55k, you should only need 200k in the Thai bank rather than making money in an investment portfolio.
2/ Money in property is as much collateral as 800k in a Thai bank, if it is shackled 7 months of the year and you still need 400k in for the other 5 months.
3/ You say the Immigration head is legally allowed to change the law. This is not a local by-law, how can they possibly have that much power, and it is often changed day to day and office to office, not by a "Directive".
4/ I suppose if you are already set up it doesn't make much difference. At 90 he might need better access to the money for medical bills, and assets elsewhere making money he can liquidate later.
Big Joke was doing a good job getting rid of over-stayers, illegal entry, frauds, scammers, drug peddlers and thieves, but this is targeting law abiding people that have pensions, cause no trouble and just want to live in the warm with their loved ones.
Much like a car park attendant, with a uniform and a whistle you own the country...