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>- Associated types in interfaces. (IEnumerable should not be forced to return a type-erased = heap allocated IEnumerator instance.)

Its not but if you call from the interface and not the implementation, it has to box because that's the contract. If you for each over a List<T> and not IEnumerable you get a stack allocated value type IEnumerator.

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Devirtualization as a (JIT) compile-time optimization is possible in an increasing number of common cases, and I can think of few cases where I'd choose to explicitly consume a non-generic IEnumerable over a non-type-erased IEnumerable<T>.

Optimizations like that is not a sound basis for an architecture, though.

Like, I get that it’s “fine” for a lot of things, but it’s very not fine in situations where you really need to guarantee that a particular operation doesn’t, say, trigger GC, like in a video game.

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