One our our engineers in a few of his sprints rewrote major pieces of Apache Tomcat, which is used in our very high TPS APIs. He fixed a bunch of the way the memory loading works, and drastically improved latency and stability of over 100 teams APIs. This guy had deep understanding of dependency injection, how java works under the hood, etc. This kind of thing might not be a huge win for a startup, but at a large corporation, you need people that can push the bounds on already well tested and well known open source software