The whole idea of this rebuild is to bypass Pasco and speed up transit times (by about a day).
The idea of connecting with the GN makes sense as it avoids the Spokane-Pasco conjestion and puts the traffic on a line that has capacity (the Cascade Tunnel limits train volume). Enviromental regulations make adding more gorge capacity next to impossible, and BNSF is in court now for extending a siding that they thought was west of the protected zone. The only real way to add capacity is to improve Stampede Pass. One of the major backers of this plan is the Washington State Department of Transportation who has promissed to help fund the improvments. If they don't manage this project any better than they have done with the PCC, I don't count on much happening besides a lot of money being spent on studies.