Well one alternative floated was to simply to demolish the unsafe viaduct with no replacement, as San Francisco did with the Embarcadero Freeway after the '89 earthquake. My preferred alternative, and much cheaper! However that wasn't a popular suggestion, probably because it would be worse for traffic (at least in the medium-term), especially given Seattle's not-great public transit. Another alternative is to rebuild the viaduct, which would be cheaper than a tunnel but still very expensive, and would probably require it to be closed in the interim anyway, because it basically needs a complete replacement of the unsound 1950s structure, not just more reinforcement. Or, of course, keep the viaduct as is and hope there aren't any major earthquakes...