(1) I-40 abruptly ends in Memphis, Tennessee DOT had to redesignate I-240 as I-40.
(2) I-70 abruptly ends in Baltimore, Maryland DOT just ended the highway a few miles short of downtown.
(3) There's a 20+
mile gap in I-95 in suburban New Jersey, NJDOT encourages interstate drivers to take the NJ Turnpike.
Therefore, I don't believe Vancouver nor Portland will miss I-5, as long as there's an alternate highway interstate drivers can use. Surprise, surprise, surprise, I-205 exists already.
Additionally I suggest many take a look how European countries build controlled access highways. Few, if any, have them running through the middle of cities. Just about all bypass the cities, going around them. It didn't hurt Memphis, Baltimore, nor will it hurt Portland and Vancouver, to route interstate traffic around them.
If you're wishing to discuss Vancouver, British Columbia, you should have place this post in the Canadian forum.
But since you didn't, have you ever ridden or seen Highway 99? It's a freeway all the way north into Vancouver where the freeway dumps traffic onto Oak Street. Again looking back at the European model, that's not unusual. As for heading in the direction north of Vancouver, there's already a highway doing so. Does it really matter what the highway is numbered?