That's a manufactured dilemma. First off, there is no such thing as "HS diploma and teaching credential". These people have fulfilled literally none of the requirements for an actual teaching credential, they're just given an "emergency credential" entirely off the strength of being enrolled in college. They ain't going to get their teaching credential until years into their career, after they complete their college degree AND a ton of other work past that. Many of them will likely quit the profession before they ever get it. But now they'll be teaching in the classroom solely off the strength of being current college students.
Second, in your hypothetical dilemma why would you know a priori that the masters teacher would give up but the high school diploma would stick it out? If you have to choose between them in a vacuum, the person with the master's degree has already proven they can stick something out and continue to work over the course of years, which the high school diploma hasn't.
And as someone who actually got a masters in teacher education and knows people enrolled in current programs, the quality of teacher education could be improved but it is NOT the first, second, or even top ten reason that school quality suffers. Teacher education programs are generally far better than the on-the-job education offered by administrators. The issue is that teachers go through their formal education learning one set of values, but then get into the classroom and are forced by state laws and administrators to do something entirely different than what they were taught to do.