[throw10920]

Aren't the necessary changes relatively "local", though?

That is - timing closure is hard, and it's a "global" problem - if your design fails timing closure, you might have to re-do the entire layout and/or design itself.

However, if you change the gate capacitances, then I would expect that you'd just have to make a lot of small changes in a lot of different (but unrelated) places - which seems like, while tedious, would be immensely parallelizable with the number of engineers, and not risk any complete re-layouts like meeting timing closure would. So, a "redesign" more than a redesign.

What am I missing here?

[Kliment]

You're missing that large feature sizes are often used because they need to handle particular voltages and temp ranges. If you change the material properties those characteristics are no longer viable with the original spacings and dimensions, so you get to relayout anyway.