We rapidly iterated before when the web was niche. When we had, comparatively speaking, very few users. Before there was a mass adoption in business.
Back then the disruption of rapid iteration and accompanying obsolescence was not a big problem. Now it's a massive problem.
Sure, one can argue that institutions stuck using IE6 (or even IE8) should get with the times and update, but the reality is that is a very costly exercise. One can't simply blindly update a few thousand machines in a company when the end result could be a few thousand people unable to work. Upgrading often has knock-on effects, so a browser upgrade may require an OS upgrade too, and every other piece of business-critical software needs to be thoroughly tested. Some software will also provide to be incompatible too, and sourcing replacements (either new licenses for new versions, or rewrites for custom software) can be prohibitively expensive.
It's those people who fear change. The IT managers that are dealing with managing thousands of machines. A failed upgrade not only has the potential to cost them their jobs but also to put a company out of business. Major change for them is terrifying.
You are right though. We can rapidly iterate now, like we did before. But we need to be careful about how we do that, and not forget those that can't come along for the ride.