There are many different kinds of Asians, and not everyone in Southeast Asia is the same:
the northern parts near China have Sino-Tibetans (Myanmar, hill tribes),
then Tai-Kadai groups (Thailand, Laos),
followed by brown-skinned Austroasiatic peoples (Cambodia, Vietnam),
and Austronesians eastward to the islands (Malaysia, Indonesia, Singapore, Brunei, and the Philippines. Also, the Cham people between Cambodia and Vietnam are their relatives).
There are also many aboriginal peoples, who are related to the dominant ethnic groups of Southeast Asia, and there are also the dark-skinned, primitive Negritos of Malaysia and the Philippines as well.
Nonetheless, some of these countries are ALREADY diverse, such as Thailand, Myanmar, the Philippines, Malaysia, and Indonesia. Not only there were Chinese people spread throughout those countries, but some of them have Indians (both Hindustanis and Dravidians) and several mixed groups.
These nations could survive a lack of diversity by expelling everyone else who does not look like them (including expats, hill tribes, and aborigines), and by building themselves like Singapore did, if they could even manage to survive independently without Western and Chinese aid.
If Southeast Asia were not so diverse, there would only be one race for every country: so for example,
Thailand will have only ethnic Thais (no hill tribes or Malays),
Myanmar would expel every non-Burmese (such as Kachins, Mons, and Eurasians),
Malaysia and Singapore would expel everyone who is not a Malay (including expats and all aborigines),
the Philippines will have only Tagalogs (no Aetas, Spaniards, Chinese, or Mindanao Muslims),
and all the islands of Indonesia (even next to Papua New Guinea) would be filled with Javanese people only.