The amount of actual threads is limited, by default to the amount of CPU cores the system has. The problem isn't the amount of threads itself, but the fact that

The amount of actual threads is limited, by default to the amount of CPU cores the system has. The problem isn't the amount of threads itself, but the fact that the threadpool will keep taking on more work, and that it's efficient enough to blow past the CPU+I/O bottleneck. You can achieve the same problem with a threadpool if your threadpool is efficient enough. Standard concurrency tools like semaphores are capable of reducing the amount of work being done at the same time. You could also simulate classic thread pools by using concurrent lists/sets/etc to replicate traditional work queues, but you'd probably lose any advantage you'd gained from switching to green threads in the first place.