Mass transit will always be worse than driving, unless traffic and parking are both really bad and transit can skip the traffic somehow. This is because of the last-mile (and first-mile) problem and the fact it can't be truly on-demand. The only way to solve these issues is with some system that starts and ends in a personalized trip, which would ironically probably be more restricted (it would probably have rails and fences)
Also, most mass transit have similarly restricted areas to roads, for example you can't walk on the light rail in seattle any more than you can walk on the road it's built next to.
Idk Chicago's transit is really good. It's getting crowded in some places, but still great.
It of course has an advantage - it's been around so long that the city has grown around major transit lines, so the things you want to do are all grouped around & easily accessibly by transit.
If you take a city like LA or Houston that has zero centralization and just slapped transit lines down, they would have trouble servicing everything and average person wants to do - maybe you can get to work on the line, but not the gym or grocery store.
Still, it would reduce car traffic, and over time it would encourage the desired grouping around transit lines that is part of what makes the transit so helpful in the first place.
I don't think it could ever fully replace cars in most places in America, as they are so spread out and many people will still want to travel between cities by car. However, even just getting most people to get to their 9-5 jobs via transit instead of car would be a massive reduction in congestion & pollution.
Living in a city that has grown around 100+ years of rail infrastructure, mass transit is slightly better than driving (1 hr MT x 1.5 hr+parking car). Sure, there's some walking, but with at most 30-minute tick on suburban rail and under 3 minute tick on city rail, a little planning goes a long way. (Also, car gets about 5x the cost per km, even before the public transit long-term ticket subsidies apply)
Also, most mass transit have similarly restricted areas to roads, for example you can't walk on the light rail in seattle any more than you can walk on the road it's built next to.