Let‘s hope it‘s just an initial limitation and we‘ll get some kind of explicit but still safe opt-out. Otherwise I just can‘t see how this can succeed as it would otherwise fully rely on the computation speed of the simulations backend. If it‘s not going to be upgraded consistently, there will be no way to improve the performance of a simulation. Furthermore there doesn‘t seem to be that much we can do on our end to optimize certain use cases, as some will still only work better in a true parallel environment.
Take a look at any mobile applications. We should only perform lightweight or UI tasks on the main thread / queue, and off load other tasks to background threads, otherwise this will likely cause hitches and bad UX. Games aren‘t that much different.