A contrast argument for the redundancy of 'exists'

In Existence: A Contrast Argument Defeated  Bill Vallicella considers an argument of Peter Lupu:

1. If a term 'F' has an explicable content, then there must be items to which 'F' does not apply.
2. There are no items to which 'being' or 'existent' does not apply.
Ergo
3. 'Being' or 'existent' does not have an explicable content.

Bill rejects this claiming that (1) is false: 

The following consideration suffices to refute the first premise. Since everything is self-identical, it is true to say of any particular thing that it is self-identical. But 'self-identical' is not rendered either without sense or content by the plain fact that nothing is self-diverse. Or consider the proposition that every event has a cause.  Suppose it is true.  (And suppose that everything at bottom is an event.)  Then every event has the property of being caused and no event lacks this property.  But it does not follow that we cannot ask what it is for an event to be caused.  The different theories of causation would be answers to this question.

It's interesting that the terms that Bill proposes as counterexamples to (1), being self-identical and having a cause, are similar in character to exists, in that, though each has sense and content, to be told that X is self-identical or that X has a cause, or that X exists, is singularly uninformative, simply because everything falls under these concepts.  The only way to convey the meaning of self-identical, etc, is to explain it in terms of already known concepts.  To try to do so by demonstrating a number of self-identical things followed by a number of non self-identical things, in much the same way as we learn to distinguish red from non-red, is hopeless.  For there are no non self-identical things and hence no contrast for a mind to latch on to.

This suggests a revision of Peter's argument:

1.  If a term F has learnable sense then either

i.  it can be defined in terms of already learned concepts, or
ii.  it can be grasped (with high probability) by encountering sufficiently many contrasting examples of things that are F and things that are non-F.

2.  Exists has learnable sense.
3.  There are no non-existent things to contrast with existent things.
4.  We learn the sense of exists by its being defined in already learned terms.
5.  Exists is redundant.

What can these 'already learned terms' be?  I suggest they are the terms some and there is which figure in the machinery of quantification as opposed to forming predicates.