The problem is that, in practice, many of those thread contexts are holding open resources like DB connections or file handles, and the costs related to those m

The problem is that, in practice, many of those thread contexts are holding open resources like DB connections or file handles, and the costs related to those mount pretty quickly. This is why actor models are good IMO. They are based on the actual resource model, rather than the developer's mental model. That is, until the developer learns to change his/her mental model.