There are PLENTY of qualified subject teachers in Korea.
It might be a little subjective about what is considered "plenty".
As many already know, there are also Korean contract English teachers that are worried about getting cut from their jobs too. Apparently, a lot of full time teachers don't like the contract workers because they do substantially less official work than they do. To me, I don't know if that is really a fair to say, only because I feel like the school teachers now aren't even suppose to be doing the bulk of the administration work. It seems like just over the years, it was kind of pushed to them and stayed as a thing. The school should really just have a bigger full time administration staff handling school paperwork, and let the teachers focus on developing better lessons for their kids. The biggest complaint that I always hear from teachers is that they are consistently overwhelmed with senseless paperwork completely unrelated to teaching.
If the system is not going to change in a way that gives "full time" English teachers more time to effectively master English to then teach their students, then I don't see a good reason to axe the contract teachers either. I think most would agree that the average English ability between a contract teacher and your average full time teacher that happens to teach English can be quite huge. At least for me, I probably catch a regular teacher make like 5-10 times more mistakes than a contract teacher does. Certain times, the English teacher speaks zero English, but is just given the job because she's enthusiastic about it? So, it does become a concern about how "accurate" students would be learning English if GETs and contract teachers get eliminated.
I personally have been here a good number of years myself, jumped from school to school like many others, but I haven't gotten the impression that "qualified" English teachers (full time ones) are very plentiful at all. If you are talking about the contract teachers, I'd say in the 70-80%+ have a very good mastery and make minimal enough mistakes for elementary and middle school. But even then, its not like the majority are like EBS TV level teachers in terms of fluency. But again, regular full time teachers that just happen to teach English...? Well,let's just say... I wouldn't feel very comfortable letting my children learn English from them, but maybe that's just me.