An obvious counterexample to this which speaks to the power of abstract art as a memorial is the Vietnam Veterans Memorial by Maya Lin which is an incredibly powerful piece of abstract public memorial art, and very near the Lincoln memorial so well worth a visit for anyone who finds themselves in that part of Washington DC. I personally admire Lincoln greatly but of the two I found visiting the Vietnam Veterans memorial a far more moving experience.

Secondly I would say the fact that they provoke thought about what they represent (rather than say Mount Rushmore in particular which is a profoundly superficial public monument) is precisely part of the value of abstract art. The Lincoln Memorial says Lincoln was a great man. OK cool. Anyone who has studied American history knows that.