i would understand if it's a thread of someone asking for help or recommendation or maybe talking about current viral things. then yes, necroing those are bad.
but people just assume that replying to an old thread is always a necroing.
so if a thing has been talked about before then you're not supposed to talk about it again
like, seriously?
Necroing is only bad if you have nothing to contribute to the thread.
For example, if you pick up a months old thread and start arguing with someone about their profile picture or because you disagree with a tangent they made. You should just have started a new thread entirely, or sent a private message.
If you actually have an idea that wasn't mentioned and directly relates to the original post, it's probably fine. It might even reignite the discussion. If starting a new thread with what you want to say would get people complaining at you for repeating old information, then necroing may be the better choice.
(damn character limit)