First of all, there is no homogeneous BC.  There is the lower Mainland and Vancouver Island, and there is everywhere else.

IMO how the Liberals fare in 2019 in the lower Mainland and VI depends entirely on who the NDP elect as leader and how they look coming out of their convention.  I think it is inevitable that Trudeau's Liberals are going to lose votes but those votes will move to the Reformacons only very reluctantly and only if the NDP are just too unsavoury.