What I don't comprehend is how someone can know of all this and then still stay "you know what? fuck namespaces and PSR, we are doing it old school!" and come up with arguments like "I/O overhead is costly in one class per file scenarios"[0].

These people will be the death of all of us.

[0] https://www.sellvana.com/blog/on-composer-psr-and-namespaces