There is no rule about that. But I bet, no one will get another DTV after the first one. There are even rumors that Thailand will cancel this type of visa within the next 3 years. The rumors come from Immigration officials, not from girlie bars.
not really challenging. I do my retirement visa extension in 30 minutes at our immigration office. House rental contract and 800,000 Baht in a Thai Bank account is all I have to provide, plus the application Form.
All these long term stayers on a DTV visa come to Thailand because it's cheap to live. A lot cheaper than in their country. I doubt they spend a lot of money. And they manage that their taxable income is very low. Thailand benefits much more from retirees and their pensions.
I guess US Americans will have a hard time getting a DTV visa after some Thai nationals have already been deported from the US even as green card holders. This US policy has not gone unnoticed by the Thai authorities.