Go try, as an American, for example, to start a business or buy land in Thailand. Sure it can be done, but there is still a real sense of sovereign control there. It's one thing for their leaders to be kowtowing (as they have been since the US abandoned our bases there) to the PRC, but quite another to become a true vassal state. Push comes to shove, the Thai royals, as well as the military, will not go that final step willingly. There is to be no Anschluss. Force will be brought to bear.
Actually it is very easy to start a company in Thailand. There are also a number of ways for a foreigner to own land in Thailand although there is no reason to go through the trouble since land can be leased almost indefinitely. The laws in Thailand are much less restrictive than in most Asian countries.
Push comes to shove, the Thai royals, as well as the military, will not go that final step willingly.
Of course, there is no reason for them to. Thais are quite skilled at playing different interests against each other. It is a large part of the reason Thailand is independent today.