In addition to ESL online teaching, there are also many opportunities for more “formal” online teaching in universities and even primary schools. For example, online higher education has become pretty well established and is provided by both totally online universities and as part of mixed programs of online and face to face courses provided by many traditional universities.
Almost all American universities provide some courses online. I have been teaching online at a variety of universities around the world for going on ten years now, teaching undergraduate and graduate level courses as well as acting as a dissertation chair and committee member.
Most of the work is part-time and the pay is the same as adjunct faculty members get anywhere in the world, while not high by Western standards it does provide a pretty good standard of living for those of us who mainly live in developing/low income countries like Thailand. While I most often combine online teaching with more traditional employment, I almost always make more money from my part-time online teaching than any full-time job in a developing country (unless when working at a Western University in China or other neighbouring country).
My online teaching has paid for my two children to attend university and paid for the house we recently bought.
Also there are opportunities for qualified teachers working with students in the USA being “home-schooled” although I don’t have any specific experience with this.
Does online teaching work? There is extensive research comparing the outcomes of online versus traditional university education, without coming to a simple answer. It seems for some people at some times, online education produces as well or even better results than traditional education, while in other people in different situations the outcomes might not be as positive.
But, if some students think it is worth paying for, then the market says it works.
I don’t have any personal experience in online ESL teaching, but I doubt online ESL teaching, due to the abundance of competition, will ever be a guaranteed path to riches, but I suspect for many teachers it could be a decent supplement to meagre ESL wages and even an alternative for a few people to having a traditional ESL teaching job at a school.