• Sometimes there's already a newer, still-alive thread on the same subject (usually created by someone who didn't know there was already a thread on the subject, but sometimes created by someone who "wanted to avoid necroposting" and so intentionally created a new thread.) You shouldn't be posting in thread 1 about topic X, if you could instead be posting in thread N about topic X.
• The older thread probably has a bunch of context given within it that's no longer relevant, and will just confuse the people trying to comment in the thread years later (e.g. requirements that are no longer valid or are already satisfied; requests/advice given in terms of architecture that's been refactored away from, or tooling that no longer exists; etc.) These threads are essentially "rotten" — only of value to a cultural anthropologist, not for driving present goals. Better to avoid reviving such a "rotten" thread, and instead to create a new thread on the same subject, where you can provide a fresh context that doesn't conflict with the previous "rotten" context.
IMHO, if comment/forum systems had richer semantics, the ideal would be to have "threads of threads" (meta-threads?), where repeated discussions on the same topic exist both as distinct threads, but also viewed in the context of a larger ongoing meta-thread (sort of like threads within a channel on Slack, but where the "channel" is still just one discussion. Maybe closer to a "user story" issue-tree-node in Jira, that has individual issues nested below it.)
As it is, frequently maintainers use some microformat or issue-tracker annotation to mark new issues as dups of old issues as a reason for closing them. (But I'd honestly suggest that that's backward: usually, the old issue is "rotten"; so a new issue should be allowed on the same topic; and moderators of the forum should go back to old, "rotten" issues and mark them as dups of the new issue, to allow people who find the old issue to follow the link to see the newer discussion on the same topic.)
• The older thread probably has a bunch of context given within it that's no longer relevant, and will just confuse the people trying to comment in the thread years later (e.g. requirements that are no longer valid or are already satisfied; requests/advice given in terms of architecture that's been refactored away from, or tooling that no longer exists; etc.) These threads are essentially "rotten" — only of value to a cultural anthropologist, not for driving present goals. Better to avoid reviving such a "rotten" thread, and instead to create a new thread on the same subject, where you can provide a fresh context that doesn't conflict with the previous "rotten" context.
IMHO, if comment/forum systems had richer semantics, the ideal would be to have "threads of threads" (meta-threads?), where repeated discussions on the same topic exist both as distinct threads, but also viewed in the context of a larger ongoing meta-thread (sort of like threads within a channel on Slack, but where the "channel" is still just one discussion. Maybe closer to a "user story" issue-tree-node in Jira, that has individual issues nested below it.)
As it is, frequently maintainers use some microformat or issue-tracker annotation to mark new issues as dups of old issues as a reason for closing them. (But I'd honestly suggest that that's backward: usually, the old issue is "rotten"; so a new issue should be allowed on the same topic; and moderators of the forum should go back to old, "rotten" issues and mark them as dups of the new issue, to allow people who find the old issue to follow the link to see the newer discussion on the same topic.)