The Internet's designers had not heard of YAGNI. They thought ahead a great deal. They even had features for PMTUD long before anyone sat down to figure out how to do PMTUD.

That's because the Internet's designers were smart, experienced, and visionary, but they didn't have infinite resources, so they did what they could, and a lot of what they could was just a sketch for others to color-in.

We should get so lucky again!