One problem with long term projects is advancement on technology that would had made a faster, sturdier, cleaner or whatever the original project if it started

One problem with long term projects is advancement on technology that would had made a faster, sturdier, cleaner or whatever the original project if it started from zero using it. We send a mission to Alpha centauri that would take, lets say, 100 years to get there with the current propulsion technologies, and in 30 years we discover a new one that would take 20 years to get there. Or the same for big buildings (I don't know, arcologies) where new materials and techniques would turn obsolete how it started if it was long enough in the past. And not only new ways to improve, but different priorities or things that are out of our control that happen during its development. What if the land where the The Line in Saudi Arabla is being built gets permanently flooded because sea level rise or the entire region becomes unlivabable because frequent high enough temperatures in the region? Even social trends may turn something built for the future obsolete, there are plenty of ghost megacities in China. Both acceleration in technologies, and acceleration in changes outside what is the construction by itself (climate, society, etc) can turn into (potentially hamrful) waste decades of effort.