[sokoloff]

Only if every part of your system is perfectly horizontally scalable. There are very frequently some singleton aspects of a system, perhaps the most common example being a single central data store with a fan out of web servers. The web servers scale by adding more, but it's an architectural change to add a second DB.

[blackcat786]

Exactly. There is always a "deployed system limit" even in horizontally scaling DB layers as deployed with sharding etc.