“It is really a practical question – how much of including non-Torah material will destroy Jewish life as we know it..”
Its really not a serious question. Without having minimal language, math and science skills, young men and women coming out of yeshivos and BY’s will be functionally illiterate, unable to perform basic functions of paying bills, booking travel arrangements, communicating with medical and service providers, etc. You cannot keep the next generation in a cave forever.