So yes, the widening of the highway makes the suburb more valuable, but the second effects make the city spend more resources on parking and makes the streets worse for anyone nearby. It's no picnic for the suburb either: A car-centric infrastructure basically bans anything else. That would be fine and all, if ultimately it produced more value than the alternatives. But look at the parking lots that were designed to never get completely full on black friday, back when it was actually a time where people did local shopping. Nowadays it's all wasted infrastructure, expensive to redevelop. If you thought that the commercial real estate office in dense cities was in trouble, don't look at the one in suburbs.
So it isn't really the highway, but all the things that relying on the highway leads to.
The problem is generally selling capacity expansion as "reducing congestion" when it does nothing of the sort. That, and the capacity gets used so quickly that you are very quickly spending yourself into a hole trying to build ever more highway capacity. The thing about a train is that it's very hard to make it so crowded the service degrades due to congestion, at least not within the first 20-50 years.
This black hole of money needed for highway congestion then starts pulling away from other municipal and transportation funding needs.
And, more than not being a problem, the highway filling up immediately is actually a benefit. Because more people are able and are choosing to travel, to accomplish whatever they need and want to accomplish.
Granted, the externality costs of all that car travel are high. But that's almost a separate issue and one that we really should be solving by a better accounting of costs and additional taxes. It's a shame that won't happen.
Traffic speeds generally have a floor of how long the equivalent public transport journey is. (If the public transport journey is faster, people will use that instead.) The issue now is that American land use is so out of whack it’s hard to deploy transit widely enough to make that true for the majority of trips.
Downs-Thomson Paradox is also interesting here and relates to your second paragraph. In a built out environment, transit trips and car trips will take roughly the same amount of time, door to door. Transit advocates have used that finding as a way to complain about road projects, but I think it actually is a much more interesting finding that suggests a more multi-modal approach to planning. I wonder how relevant or irrelevant it is for non-built-out environments.
Usually the issue with suburban transit investment occurring over urban ones is that urban areas are so weak they cannot fund expansions by themselves, and if you want the suburbs to pony up they’ll get their pound of flesh
The alternative isn't eliminating those trips. It's taking the trillions of dollars we waste and spending it on an alternative, more efficient modes that deliver better outcomes (travel time, cost, speed, comfort, etc)
Urbanists tend to prefer walkable cities. They want to use less above-ground space for traffic and parking, leaving more space for buildings, parks, and public squares. Highway capacity expansions work against that, because they encourage more people to drive into the city.
* driving is just less healthy than walking, biking, or walking to and from public transport
* driving causes high noise pollution and emissions; EVs will solve the latter but not the former, since above low speeds most of the noise is tires rolling against roads
* increased driving in the US has led to higher fatality rates for the people still walking and biking
* increased driving leads to lower funding levels for other types of transport since road infrastructure is so expensive, and not everybody can afford a car to avoid getting left behind