Most sublicing is just the 2-arg kind so it would not be safe to truncate the allocation even if the subslice is the only living slice because the capacity still allows indirect access to the trailing elements of the original slice. This optimization would only be truly safe for strings (which have no capacity) or the much less common 3-arg slicing (and Go would need to have a compacting GC).
Of course, the language could also be changed to make 2-arg slice operations trim the capacity by default, which might not be a bad idea anyway.
Yeah I understand why they can't do it for backwards compat reasons. Considering they had the forethought to randomize map iteration order it's a big foot gun they've baked into the language.
Of course, the language could also be changed to make 2-arg slice operations trim the capacity by default, which might not be a bad idea anyway.