I believe I agree. Density doesn't justify inaction or continuing along the path of inefficient individualized transit, so it shouldn't be used as a red herring, but it's an important consideration that affects how people will use public transit. Regardless, there should be far more investment in transportation, because places in the US (among other places) that could be practically served by public transit still aren't, and you're right that investment begets clustering around transit hubs so it is necessary even before it seems ideal.
As I said, there are quite a few more options nowadays for the last-mile: shared bikes, electric scooters/bicycles, or even Uber/Lyft/rideshare. This should further increase the viability of public transit even in areas of the US in which it wasn't previously viable.
Yet we still had perfectly functioning public transit.
Sure, there are definitive advantages that arise over time once you already have it, such as clustering of shops near transit hubs instead of spread out all over the place, but it still works.
It boils down to willingness to invest. You need to commit to ensuring sufficient frequency and routes for years to change habits even if it operates at a loss.
This is the real problem. You need to be prepared to put the service in before the demand is sufficient to really justify it, because the long term social impact justifies it. Do that, and it shapes the landscape. I mean, the train did that in the US too when it first arrived: entire towns sprung up around access to transit. The convenience of cars would make it harder to cause a reversal, but it's just a matter of which benefits you need to create.
So it's down to political will and lack of public support. The density is a red herring.