Like anything else, it's about tradeoffs. This series of planning decisions had a ton of downstream consequences, both positive and negative (and I'm happy to enumerate plenty of examples of either). I certainly don't enjoy the amount of traffic that has come with the city's growth --- I'm a lifelong Portlander, and have watched it happen --- but I firmly believe that more freeways would not solve the problem. Induced demand is a thing, and there is also geography to contend with. For example, one of the biggest traffic choke points is the freeway coming in to downtown from the western part of the metro area; since the 80s, there have been several major (one might even say "meaningful") projects that have widened it about as far as it is possible to go, but at this point there's literally nowhere else to put another lane. Of course, part of why it's a choke point is that after the tunnel it immediately intersects with another ill-sited freeway --- one of the last ones built before our "revolt," and one whose siting and construction caused logistical problems that the city is still dealing with, fifty years later.
Anyway, it's complicated, but the local government "betting the future on light rail" is not the reason traffic has gotten as bad as it has. Population explosion, geography, and seventy-odd years of path-dependent decisions about freeway placement and urban planning are the reasons.
