Let’s say we started a project to send a probe to Alpha Centauri. With the entire world dedicated to the task, the probe could probably be launched in a decade or two, and would reach its destination in maybe 30,000 years.

Or we could forget about the probe and focus on developing better propulsion technology. That would have immediate applications, but could also allow the probe to reach its destination orders of magnitude sooner.

I don’t see any reasonable motivation for generations-long megaprojects until technology has well and truly stagnated.