If I understand your argument correctly, it is certainly possible that as America gets more diverse it (the Democratic Party) could become more progressive.
Where I could see pushback is going back to the demographics = destiny argument. That was never about whether these new non-white folks would be more progressive vs moderate Democrats, it was simply they would vote Democratic.
I'm not saying it can't happen, but the fact that many Hispanic and Asian voters have proven less predictably Democratic than expected is one reason I'm skeptical that demographic change alone will push the Democratic Party in a more progressive direction.