In the fast-moving open-source world, programs can come and go quickly; a tool that has many users today can easily be eclipsed by something better next week. Even in this environment, though, some programs endure for a long time. As an example, consider the PostgreSQL database system, which traces its history back to 1986. Making fundamental changes to a large code base with that much history is never an easy task. As fundamental changes go, moving PostgreSQL away from its process-oriented model is not a small one, but it is one that the project is considering seriously.
I think we’re starting to hit quite a few limits related to the process model, particularly on bigger machines. The overhead of cross-process context switches is inherently higher than switching between threads in the same process - and my suspicion is that that overhead will continue to increase. Once you have a significant number of connections we end up spending a lot of time in TLB misses, and that’s inherent to the process model, because you can’t share the TLB across processes.