This particular market seems to be falling off fast nowadays. Years ago you saw many private language school chains in shopping malls and office buildings, often catering to Thai office workers who would come and study English in small groups when their working day was done. These private language schools are still around but they are nowhere near as prevalent as they once were.
Where have the Thai adult learners gone? The word is that many of them now study online instead of battling their way through rush hour traffic to study English at a language school. Definitely a sign of the times.