I would be very hard to write correct software involving threads without that guarantee. Most threaded programs are using lots of private data, and imagine if a

I would be very hard to write correct software involving threads without that guarantee. Most threaded programs are using lots of private data, and imagine if another thread could just randomly stop on your private data while it was trying to do a nearby write? Maybe you could try to formalize the idea of "keep private data away from data on other threads" or something, but this seems very difficult (in practice, private data is private as an emergent property of the whole application, so is not easily identifiable), and you'd have to give up on a lot of things the existing model offers in terms of write-independence.