alt Hacker NewsMore load does not just mean needing to increase capacity, which would be simple for Microsoft to do.
Additional load reveals limits and breaklines that were never known before - your temporary integer IDs are suddenly too small, you reach the maximum file descriptors in a Linux distro, your hash function suddenly has repeating conflicts, your DB index is suddenly oversaturated.
Having massive exponential growth of your traffic would probably require constant refactoring to meet the ever changing system limits.
A good rule of thumb I once heard (from Jeff Dean, I think) was that (in a complex system) every 10X you go up in scale you need to re-consider basic design elements, trade-offs, etc, and probably change some of them.
It's hard to design something (large and complex) for much more than 10X scale with only HW capacity increases.