I live in Thailand. There's less incentive for the husband to ditch the wife, because the marriage visa is very crucial for him being able to stay in the country. There's no guarantee that the mistress can get the paperwork cleared, and no guarantee that she would be as compliant as his current wife. The current wife is a known quantity, and if she's willing to hire a mistress, then there's really little reason to switch up.
It's a well-known, but not overly-emphasized reality that marrying (or even just dating) a foreigner is a pathway to a better life. I'm under no illusions that I'm able to bat way out of my league romantically due to my foreign status. I'm common law married with a Thai woman, and we intend on getting legally married soon. I actually love her, but the visa would be a very nice bonus. Right now, I'm on a more expensive work visa. My wife is also compliant with some of the extramarital stuff. We've only discussed it a few times, but she understands that there are urges, and we just establish ground rules. It's a matter of looking the other way, because I'm faithful and with her like 99% of the time, and I genuinely love her. But you know....urges.
There's a certain amount of guilt about this, at first, but this is really the way it works for many expats. You have a good woman at home, and a random hookup or steady mistress on the side. You open a business with your wife. She keeps a majority stake in the company, because of Thai laws. You provide support and security for her and her family. If things fall apart, she at least gets a business that she can grow to support her and her family back in her home province. The occasional (or regular) hall pass is a fair trade.