Your analysis seems to draw on the idea that in the future, there will be more potential progressive voters, mainly in minority groups.

Now, it is dangerous to judge these things from across the ocean, but I always understood the US already has a large reservoir of mainly poor people who do not vote. but that has not lead to a (strong) political group more progressive than the current Democratic party.

I suppose that poor people of any ethnic descent would be at least as reliable as progressive voters as minorities, given that they stand to benefit from progressive politics, while this is much less given for minorities in general. Besides, your statistic showing young people to be more progressive can be misleading: it is widely assumed that people get more conservative as they get older and calmer ( and richer and more settled with more to lose). So your potential electorate might not be that progressive anymore in 20 years time.

So, if there are currently forces in politics that prevent your desired progressive party even though the electorate for it might be there, why do you suppose this will be different in the future?