There are some advantages in some domains for expressing things as an existential. In particular, you get implicit conversions from values that conform to the existential's protocol to the existential type. In an earlier version of the Python interop design, PyVal was an existential. I ultimately moved away from that, but I wouldn't be surprised that it would be useful for some other application.
To be clear though, I'm just saying that there is a thing that exists and it could be useful for some cases. I'm not saying that this is strong motivation.
We debated this at length in the core team meeting, and my interpretation of the discussion was "yes, this could be modeled as a protocol" but "this would mislead people". The problem is that being a protocol would naturally lead people to think that they could use it (e.g.) as a generic constraint, but the underlying model does not support that. If we provided it as a protocol, people would see it as a deficiency that the implementation didn't support that and that they would keep asking for it. Given the implementation approach, these approaches would be "wrong", but they wouldn't be wrong for asking, given how protocols fit into Swift.
Given that, and given that it could also be very reasonably modeled as an attribute, it seemed better to model it as an attribute: doing so doesn't lead people into a path where it feels like an uncanny valley where it is somewhat like a protocol but not really. It really is compiler magic, and an attribute is a good way to model that. The thing that eventually really convinced me is the fact that you'll be able to define a protocol with this attribute on it (assuming the protocol provides the subscript) and that THAT protocol will work properly with generics and existentials of desired for a specific application.
While there are reasonable arguments on all sides, this feels "just right" on balance. I'm sure that others will feel different of course, but given that these spelling decisions don't fundamentally affect the expressivity of the proposal, it seems like a good place to land.
-Chris