Even regular Python code expects to run in the main thread, because only the main thread gets KeyboardInterrupts. But that doesn’t stop us from using separate threads for executing code and for running the kernel/zeromq/tornado stuff; it just means we have to put the kernel/zeromq/tornado stuff inside a child thread, instead of vice-versa.
I guess this is basically suggesting that we have separate kernels python, python-asyncio, python-trio, etc.? That could work, but I feel like people would find it much more pleasant to pick the eventloop with a %magic command, instead of having to pick when you start the kernel?