> The fundamental reason, I think,
> is that a Fixnum object (upon which such an operator would operate)
> should never be mutable.

Yes.

> The x+=1 operator emphasizes that a new Fixnum
> object is being assigned to the applicable variable.

No.  (To me, anyway).

I can't say I associate "new object being created" with "+=".  Now I know,
deep down, that it's syntactic equivalence to "= .. + ..", and THAT
emphasizes "new object", but on a more gutteral level, "+=" means "new
object creation" no more than "++" would.

Maybe that's me though.