I have sympathy for what you’re attempting to accomplish and also what @downey said.
Of course, we’re coming from mailing list culture where mailman archives persist forever, so that has set the expectation, while I realize a forum is a different beast.
What if instead we just archived out of date topics?
And made it so that archived topics were hidden from search by default?
And later when search gets fancier, a search parameter could allow including archived topics in the results.