There are a multitude of arguments to be made, pro and con, about a new bridge across the Columbia for Interstate 5. There are a number of competing arguments to be made regarding extending light rail to Vancouver. Arguing that we are too far down the current path to reverse course is erroneous at best and ridiculous in most of the cases where it has been offered. If the design fails to meet its intended purpose, then it is never too late to stop, regroup and redesign. It is far better to waste a couple of hundred million dollars than it would be to waste $4 billion on the wrong bridge. It's too early in the process to say it's too late to build a higher bridge or a bridge with or without light rail.
What the planners should do is take a deep breath and do what they should have done at the outset. They should determine what the requirements are for a crossing that meets the needs of all users; then and only then, design or modify the existing bridge design to meet all of the needs. There is only one chance to get it right. Proper planning wasn't done at the beginning and still hasn't been done, yet.