Yes changing the global template would involve regenerating all 100,000 pages in most static site generators. The only exception being those rare ones which off

Yes changing the global template would involve regenerating all 100,000 pages in most static site generators. The only exception being those rare ones which offload template rendering to client side (there's a couple that I've seen). Still, some static site generators like Hugo, are built for bigger sites specifically by being able to generate massive amounts of pages very quickly. Even in your 100,000 page example a site probably would be completely rebuilt within a second or two.