I had a think about that.

I think the Democratic Party became more progressive and “enlightened” at the same rate local politics lost its ability to do significant changes.

As activism across the political spectrum diminished in the 1990s, and as local grassroots movements dried up due to population movements. People turned to a higher yet inefficient source of power; state and federal democrats . It was about that time the party started to rebrand itself to seem more useful to these people

Most people who have grandchildren will remember that mainstream democrats were a bit scary back in the day. Certainly not any sort of pressure value ( in Texas even in the 90s they used to hold political meetings during hog hunts)

In many states, particularly the south, the older Democratic Party was completely replaced by others who had none of the connections, wealth or power.

I think to a while, after that, the democrats were a pressure valve, like you described.

But now we are in a time of political stasis. And nothing would change tomorrow if the whole party went away. At this time the only thing keeping us in check is us, the population, who is challenged to do anything