Yeah, I don't think it's impossible to fix the US. Or frankly anything else. But possible and practical are very different things. It's possible, but it would take a massive amount of time, money, and political will that would have to basically come out of nowhere. It's not practical to expect that in the foreseeable future.

But just because we can't "fix" it doesn't mean we can't at least make improvements. Fixing isn't really a binary. Even if we can't do much for existing suburbs with their massive sprawl and absolutely nothing you can walk to, we can make city centers more walkable and expand public transit within them. We can avoid building more suburban sprawl. We can make walkability an important criteria for new neighborhoods. Even if we can't fix things for everyone, we can at least improve things for a significant number of people and that's absolutely doable.

My Canadian city has been improving in recent years. We got an LRT in the past 5 years or so and it's amazing if you're on its corridor (and that has prompted more and better development along it). We've added a bunch of bike lanes (proper ones with concrete barriers), removing car lanes in some cases. In our downtown, the sidewalks got expanded and street even closed to vehicles to be pedestrian only (and part of it is full of picnic tables and stalls for festivals). We still have a long way to go and the city still down have a bunch of suburban sprawl, but it is getting better and we should not give up on improving it.