It will certainly have it's limits. I don't know whether tokio spawns new threads if it detects all others are used up - likely not that this point of time. How

It will certainly have it's limits. I don't know whether tokio spawns new threads if it detects all others are used up - likely not that this point of time. However work-stealing at least allows to mitigate the impact of some accidentally blocking code. E.g. if a syscall is made that takes longer than expected - or if a library holds a mutex for longer than necessary. It shouldn't be used as an excuse for simply blocking everywhere in an async task executor - but it will to reduce the impact, and give developers some time and wiggle room to improve the associated code.