> Then, six months later, you add a new field: shippingAddress.

The problem isn't that you need to add a new field, the problem is that six months ago your design wasn't sufficiently thought out.

Ie, modifying event-driven systems is hard because designing them to avoid later modifications is even harder.

You can never sufficiently think things through because the future is necessarily uncertain. Hence you need to be flexible and hence the actual contents of the article.