> The hard part of building a CDN is to know when you need it. 99.9% of all websites with CDN do not need it.

What exactly leads you to believe you can tell what 99.9% of all websites need?

Unless you believe 99.9% of websites are only accessed by your upstairs neighbors, CDNs provide a couple of important business and operational advantages.

Just to illustrate how wrong and misguided your personal assumption is, CDNs are primarily used to cut down latency, which in accesses from other regions can easily go beyond 300ms. Unless you somehow think that it's ok for your users to be subjected to a bad user experience, a basic CDN service is all you need to employ to lower those latencies by an order of magnitude.