The Magisterium is not 6 or 7 thousand bishops under the pope, it is the authority Christ gave to His Apostles and their successors, the bishops to teach in His name, to bind and loose. This gift or charism has been enjoyed by all St Peter's successors and the bishops in communion with him from day one.
In the unlikely, I would say almost inconceivable, event of all Catholic bishops dying at the same time, then I would have to say that without any bishops at all then there can be no Church. However, on account of Christ's promises to be with His Church until the end of time and where 2 or 3 are gathered together in His name, He is there in their midst, my Faith tells me that God would not allow this total catastrophe to befall His Church.
I do not expect you to agree with any of this but this is what I and millions of Catholics of my generation were brought up to believe and nothing the Church has taught since contradicts it so I'll stick with it.