I thought of that too but it's still creating a dupe. I'm imagining a long-running bug or maybe some semi-popular though obscure edge-case hard to nail down. Could wind up with a huge web of topics about the same thing.
When I was thinking about the solution of allowing, it crossed my mind that it could be a request requiring mod approval: Maybe a button "Request reopen" or even better: It allows you to reply but if it's closed, gives you a message with the aforementioned restrictions/requirements. Then, it goes to a mod queue.
Idk if that's even possible with this forum software without custom code but where there's a will, there's a way. IMO it would be worth it if a solution already exists to do it. Otherwise, maybe too much work.