You have to build rail to get this kind of new capacity. That 14,000 people per hour is eight northbound lanes (four general, four express) of traffic. Link will move more than half again that many. What would it cost to put four or five more lanes each way on I-5 in Seattle? Where would those cars go once they got into the city?
And remember, all these new light rail trips have no congestion. Those I-5 trips are often at a dead stop – but those light rail trips are cruising along between stations at 55mph.