Exactly this. You can either having pages that scroll horizontally or content that reflows. There is really no other option available when dealing with a layout that can be arbitrarily resized (edited to add: there are a couple other options such as scaling down all individual UI elements, or simply removing some elements but those aren't really viable solutions in most contexts).

Since we have collectively decided that horizontal scrolling is unacceptable in almost all web contexts, that leaves reflowing content when the window gets smaller.

I understand the complaint and I sometimes have a similar frustrations. But, what other options are there that don't introduce horizontal scrolling?