There is no good technical reason why the community version of Synapse should not include the worker implemented in Rust. A smaller, resource constrained server, such as a single board computer with 2 GB memory would benefit from a more efficient implementation.
Element seems to be adopting the open-core business model like GitLab. I don't blame them for this; they did mention funding problems recently, but the arguments and analogies in the article don't make sense.