The restarting of a 767 passenger line is not a risk to the 787-8 for Boeing.

The problem for the 787-8 is that the 787-9 is too good! On equal stage length flights, they have almost identical trip costs. So the -9 can do any flight the -8 can go for the same cost while generating more revenue and the 787-9 can go farther then the -8 when needed, so nobody is ordering the 787-8 anymore.