Because this is the LAST leg of your journey, you can dump it and nothing will happen.

zilch.

zero.

Airlines can't issue you a debit memo for failing to show up for that last leg.

Alaska doesn't have a huge issue with no-shows like American, so their system isn't actively looking for you on other flights. Go ahead and book the AS flight SEA-YVR. You will be fine. 10000%.

In my job, if Alaska had *any* tight revenue integrity systems in place (which is what this is called - Revenue Integrity) - I'd be getting notices from Alaska for employees who are flying standby that also have confirmed seats on the same flight. There are presently a few ticket sweepers (part of Revenue Integrity) in place, but its looking mostly for unticketed reservations, duplicate/similar names, watch list, etc. Its not comparing SEA/PDX lists with SEA/YVR, for example. *well, not yet.