Sir, you do not expand community engagement by removing community engagement. Content in this forum, and any forum, is already sorted by date, so arguments that "old" posts or threads are some how detracting from current discussion is spurious. Unless they're searching for it directly, the only way to find "dead" posts that haven't been active for six months is to make the conscious decision to skip over pages worth of current discussion...if there is any.
You've made it clear you're not deleting these posts and threads to somehow conserve diskspace or optimize a database or anything of that nature, all you're doing is preventing the community from accessing old content...and to those in the community who've become skeptical of WG's engagement of the community, it gives off incredibly bad vibes of trying to hide inconvenient discussion or ability to compare how the game and it's community interacted in the past to how it's being run in the present.
But I'll give a benefit of the doubt and take what you and Hapa have said, in that you saw this as simply a measure to "tidy" the forums and make it more accessible. I think numerous people have demonstrated that whatever benefits you think you'll reap from this process is outweighed by the image it's giving off, and the practical loss of valuable past discussion and community interaction.
Admit it was a bad idea done with good intentions, and make a commitment to restoring the old content, without trying to force individual authors to request individual threads. Some authors may no longer be active, some threads may simply be forgotten by those authors.