What this method gives us is the topology of the phase diagram - what lines intersect and in what relative order. The metastable extension means this. Now suppose we don't have any excess silica. Then the reaction can't go. That means we could still go to the right of the vertical curve and have the reaction on the horizontal line occur.
L can go.
The metastable extension will turn out to be important, as we shall see below. The real power of this method now becomes apparent. For any given line, all the other line labels must be consistent with the reaction represented by the line. For a sector bounded by two lines, the line labels and the reactants within that field must include all phases. The line labels cannot include phases present among the reactants and vice versa.