Nobody has suggested that you don't know about university teaching. You are obviously well versed in one specific area and that is fine.

But you do NOT know everything about teaching ESL, especially the area of YL teaching from Elementary to Upper-Intermediate. In this area you cannot teach 'descriptive' grammar; students learning a language from scratch need to have structure so they can unlock the language themselves. They need to know why we say certain things in certain ways and so as such, when teaching ESL, teaching the language how it is supposed to be spoken is the best and only way.

An example of your 'descriptive' grammar would be Welsh dialect; my Welsh friend would say "I seen that movie last night" in place of the correct past simple "I saw that movie last night". Whilst his use can be understood, it is not correct, however allowances are made when speaking to take in to account his regional interference, but in written English those same allowances would not be made. Another example would be the film 'Trainspotting' which used regional Scottish English and as such the use of subtitles were widely needed in English speaking countries such as Canada, America and Australia in order for watchers to understand the language used.

If a student, after gaining proficiency certification, wishes to take their studies further and investigate the nuances of language as it changes regionally or nationally, then fine, that is up to them and that is when how language is used in different places has value.

ESL learners, at any age or level, are not learning Chinglish, they are not learning the regional dialect of East London (for example) or the bastardisation of the English language when assimilated in to another culture. They are simply learning English, how it is spoken in either England or the USA, those are the only variants needed in an ESL classroom from A1 to C2.

To the student acquiring a new language for the first time, learning it correctly is the only way.

By saying otherwise you are confusing degree level studies with the teaching of English as a second language. They are two completely different things.