This is not to suggest that changing times might not render a given public monument intolerable. A simple test might ask: What was the purpose of this monument? What was it intended to honor? If that purpose is either historically dead and thus safely inoffensive (James II) or historically alive and still valued (Churchill), we should avoid devising rationales of condemnation.
The “why” test would doom most statues of Stalin, for instance. Those monuments never represented a shared purpose, and certainly don’t now. It might allow some Confederate memorials (such as those remembering the dead), but disallow those celebrating slavery or rebellion. By and large such a “why” test would leave public monuments in peace. Commemorations of Ulysses Grant celebrate him for winning the Civil War, not for waging war on Native Americans. Jefferson is honored for writing the Declaration of Independence, not for holding slaves.
Monuments aren’t erected in a spirit of blind idolatry. They commemorate particular achievements of imperfect people. There are other mechanisms—schools, documentaries, museums—that instruct us on the flaws of our forbears.
Rather than the “why” question, lately society prefers the “me” question: How does this monument offend me, or my community?