Trust me, we went down this rabbit hole when we designed the iterator protocol. The problem is that that sentinel object must have a name (otherwise how would you know when you had seen the sentinel), which means that there is at least one dict (the namespace defining that name, probably the builtins module) that has the sentinel as one of its values, which means that iterating over that particular dict's values would see the sentinel as a legitimate value (and terminate prematurely). You could fix this by allocating a unique sentinel for every iteration, but there would be some additional overhead for that too (the caller and the iterator both need to hold on to the sentinel object).

In any case, as I tried to say before, redesigning the iterator protocol is most definitely out of scope here (you could write a separate PEP and I'd reject it instantly).

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