Tacoma street East of the bridge wasn't gonna get any wider no matter how many lanes wide they made the bridge (which is mostly one lane each way). It would make no sense to widen the bridge beyond the width of Tacoma.
And worse, eventually PDX is going to wake up and realize they need to widen key surface streets to address their traffic hell. That will be expensive enough on it's own. But after you already blew $300M on a bridge to ensure it will always be the bottle-neck, it will be far more expensive.