Schema changes, and how they affect up-time, are an implementation detail in relational databases. It's possible to design a relational database engine that doesn't require huge amounts of downtime to alter the schema of large tables. It's all in how the data is stored.

Of course, the reality of the situation is all that matters, and most database engines at that time did not offer such features. So, I don't necessarily fault people for trying to find solutions that simply worked for their needs.