The Thai immigration website does state proof of income source, but doesn't exclude sources other than pensions. Many retirees derive income from property rentals, which I know for a fact is acceptable by immigration for extensions
I've never used an agent to pay bribes on my behalf and I've never had problems with immigration. In fact, if anything I've always experienced excellent service. Maybe they actually respect those who want to stay legal. However, unless immigration changes the criteria for the "retirement" visa, the source of the 65k is irrelevant. Immigration still doesn't officially call it a "retirement visa", it's actually a non-immigrant 'O' visa based on being over 50 years of age. Nowhere does the criteria even suggest an applicant must be retired
I don't know what will happen in the future. I always take my annual pension statement to immigration at each annual extension, but it's never been requested. With the shonky bank business going on in Thailand at this time, it wouldn't surprise me in the least if source of funds is required in the future. That'd be great, as I'd be watching those with agency accounts squirming as they try to explain the sudden appearance of 800k in their accounts!