Other reactive frameworks like MobX will react when you only mutate objects. APIs like the useState hook at least make it obvious that you need to replace the whole object. Svelte on the other hand presents this "it's cool man, just change your plain JS state like normal!" paradigm, and it would be very reasonable without being told otherwise to assume that this applies to mutating objects as well. But then it doesn't.

If re-assignment of the whole object is what I have to do anyway, I'd much rather just be given an API that's slightly clunkier but makes that fact obvious. Otherwise the behavior will be surprising and error-prone to those not closely familiar with it yet.