Imagine listening to an old musical recording on a set of speakers. At low volume, pops and cracks are noticeable because old recordings were done on records which had physical scratches that created the extra noise.
If the volume is turned up, so that the old recording is played louder, the pops and cracks will become more noticeable, and may start to take up more of your attention, and as the volume becomes very loud, the reduced experience from listening to the music with the pops and cracks may cause you to stop listening altogether, because the defects have made it unenjoyable.
Now imagine listening to a very clear new musical recording on the same set of speakers. At low volume, the sound is clear and enjoyable. At a reasonably high volume the sound is clear and enjoyable. As you approach the maximum volume, pops and cracks start to be audible, as the defects in the speakers themselves are now being displayed.
Volume causes defects to become noticeable and important.
This is why a design for a web application that is quickly thrown together may work for an initial group of users, but once the site becomes popular, the application could crash or fail to keep up with the load. The operation of the application may require many more servers to be purchased, or worse, not scale onto more servers and require purchasing an ever larger and more powerful single machine.
Horizontal growth can be purchased at slightly more than linear pricing, with a higher staffing or automation requirement, vertical growth grows exponentially more expensive, until it ceases to be possible to grow more vertically, and some horizontal scaling must be added (typically by horizontally scaling slowly with similar expensive vertical solutions).
The volume of use anything has directly corresponds to the likelihood that defects will cause a failure.
Once this is understood, and used to assist in decision making, then a solid plan can be put together for how long a defect can exist on the path to increasing volume.