Well, no matter how fast a single server is, it can't keep getting faster.
You might shoot yourself in the foot by optimizing only for single servers because eventually you'll need horizontal scaling and it's better to think about it in the beginning of your architecture.
This is far from inevitable. There are tons of systems which never grow that much - not everyone works at a growth-oriented startup - or do so in ways which aren’t obvious when initially designing it. Given how easily you can get massive servers these days you can also buy yourself a lot of margin for one developer’s salary part time.
