I used to think this way, until I tried writing more sci-fi and I kept running into weird moral quandaries trying to keep stuff realistic on a human level. I genuinely don't think there will ever be a threat that could rally all of humanity at this point. Not only because I don't believe aliens are a thing we'll ever experience, but also because everything I've seen points to people being too chaotic. Even the perfect enemy (some bugs that just want to kill us all) would have humans helping them out, a contingent of people who think the whole thing is a deep fake, and a multitude of people preying on the flawed reality of those groups and others to horde whichever resource (money, food, manpower, etc.). That's before you even get into the various well-intentioned factions that would form around a variety of "best" approaches to the issue.

I'm not even saying this in a doomer kind of way, I'm rather optimistic and believe we tend to stumble forward. I'm just saying the rally around the flag moment for humanity feels like a total fantasy.