Interesting points! My argument against using a subscript is getting weaker. I'm tempted to say that slices fall into the "term of art" category where Swift didn't really try to design the API in a vacuum, but that doesn't change the fact that they do indeed provide a view. Maybe my problem with subscripts for defaults is that I feel like it's unnatural to think of it as a view/slice in the same way I think of actual slices. But maybe that's just me.
Side note: this led me to look into UnboundedRange and found interesting comment from Dave — maybe Optional as a collection would just use the [] spelling :)