I'm just finishing up my MFA now and about to commit to an English PhD so can speak a little to this. I think it's vital to not equate an MFA in Creative Writing with an MA in English, and to view them as being very different entities. The MFA is primarily for your development as a creative writer. Although you can—as I did—take English classes, the focus of the MFA isn't the literary study of texts.
Nowadays, even having an MFA alone is not enough to get a bad adjunct job teaching creative writing. Even though the MFA is, technically, a terminal degree, you'll need a history of publications, and a book—at the least—in order to get a job teaching CW. Having just an MFA alone also won't easily let you transfer your skills to teaching English literature; but having an English PhD (as well as published creative work) will let you teach creative writing and apply for creative writing positions.
I do think having an MFA can help when you apply for English PhDs but that also depends on your various accomplishments as a writer, and whether you have a 'public-facing' practice. Because the academic job market is so bad, I get a sense that graduate programmes find students who have something 'alternative' (e.g. a practice as an artist/writer) as increasingly attractive candidates.