Then I disagree with you. I think it's more related to time in martial arts, and other characteristics that aren't based on MA or age at all. I know plenty of people currently in their 20s to early 30s, that fully accept they're going to have to put in a lot of effort and time to get good at MA (and have done so), and understand that they don't fully understand their arts. I also know plenty of people in their teens, 20s, 30s, 40s and 50s who assumed after a few years they knew everything there was to know, and gave up on their MA because of that.
It may be a difference in experiences, but mine so far does not support that idea at all. I'm also confused about why you hope it is millennial-generation specific, but I'm guessing that you just mean that you hope the next generation will not be like that, rather than hoping millennials are like that.