The point is not limited to Int though. Say you have this struct:

    struct MyStruct
        a::Int64
        b::Float64
    end

Since the size of each field is known (8 bytes), the size of the whole struct, and thus the size of each instance of that struct, is known - 16 bytes. This is crucial information for inlining, loop unrolling, copy elision, deciding which register(s) to place the object in, getting rid of indirections through pointers.. If you'd allow to subtype MyStruct, the compiler wouldn't be able to know ahead of time what the size of a MyStruct object is and would either have to box every access (introducing pointers which have to be chased on access) or defer A LOT of work to runtime. With boxing, you almost immediately lose lots of opportunities to SIMD or otherwise optimize your code, because you have to keep a whole lot of extra type information around at runtime that's just not necessary when all your objects in a e.g. Vector{MyStruct} have the same size anyway.