The article conflates a few different "slow" projects, rather than the premise which is efforts that required decades to come to fruition.

He mentions projects started long ago but are still ongoing, like the Sagrada Familia. Then there's innovations from long ago which are still being used, like Linux. Also, he includes ideas which took decades to finally be implemented, like LIGO.

In my opinion, none of these examples are particularly good at demonstrating, "What problems can human beings only solve over a very long period of time?", except for Fermat's Last Theorem.

All technology builds on that which came before, step by step. You can trace Unicode directly back to Morse Code, via various steps like ASCII, Telex, Baudot Code, etc. But the original goal of Morse wasn't to display emojis.

I'd say General Relativity might be a good example, starting with Newton's efforts to quantify the forces of the real world, ending with Einstein's explanation of spacetime. But again, it's not as clear of a problem as Fermat's Last Theorem which was a single problem that required centuries to solve.

AI may be a good example as well, starting with the advent of the digital computer. The very first scientists who worked with them like von Neumann immediately looked forward to the day of an electronic brain. It's taken nearly a century so far and is still underway.