the unfortunate things they don't tell you about You Aren't Gonna Need It are that:
a) you might actually need it though
b) the longer before you realize you actually needed it -- the closer you get to being right that You Weren't Gonna Need It -- the harder the work to unwind the assumption will be
c) being wise enough to think in the moment that You Weren't Gonna Need It doesn't get you a discount on that work
Some features really do get much harder to add after the fact - I like Simon Willison's and Luke Plant's lists of PAGNIs, "probably are gonna need it" features
Sadly none of these lists have the one true example of PAGNI: undo/redo, which is straightforward to solve with the command pattern if you think of it ahead of time, and pain otherwise