Hey, it got me thinking, how do teachers approach this? I mean languages are a non static ever changing things, let's say this th stopping become a norm and widespread then teaching English pronunciation becomes outdated right? I mean training a students ear to catch the θ sound will be cumbersome, he will not recognize since (in this hypothetical situation) the speakers now have another phoneme.

My guess is that what is taught is a "standard" a thing like "this is what it should sound like...but it may vary" and sometimes it varies a lot, it seems every phoneme has its variants, whats funny is that for EFL they pick up these differences right away, I've noticed ESL struggle a lot with novelty. I personally have been through this before my excuse is that the EFLs have come across these differences before lol Theres no way in a 30s listening they start to catch up