There's no such thing as universal millennial optimism.
It's a made-up term that describes, at most, a small percentage of humanity, not some global phenomenon. It's better to change it to "middle-upper class rich western world optimism" and then it will make some sense.
It's like assuming all boomers were hippies when that only applied to the Western world, and even there, most boomers weren't hippies.
I'd like to take this opportunity to say that this sub is filled with naive, inaccurate, or completely made-up generalizations about generations, which often don't even apply to Europe and seem to apply only to Trumpistan.