Coming from my background, I'm absolutely convinced this happens. But I doubt it is a major problem. I'm sure if it was, that the Thai authroities would get wind of it and deal with it. What we're talking about here is taking a sledgehammer to crack a nut. For example, the Thai authorities could ask the Embassies that don't currently notarise docs, such as the US, to do so.
Ultimately, most farangs will have an income greater than most Thais, even if it doesn't meet the required amount and they spend that money in Thailand and pay taxes such as VAT which go into the Govt. coffers. If this plays out that more expats leave, the Thais themselves will suffer.