For the past 3.5+ years I thought this kind of discussion (aside from the 'new' OMNI team) on any action of moderators was taboo.

Since I believed that was the case, I've only been in contact with the appropriate volunteers if there ever was an issue.

Seeing as this is okay to discuss or express an opinion - I think that it is the moderator's prerogative, based on their experience and judgment, to merge as they see fit.
It seems to be common practice AND part of the TOS that really old (24mth+) threads not be bumped or merged.

For some sparse threads it might make sense to do so, but I agree with the sensible suggestions above that threads dormant for >24mths be tagged as archived/closed.
People who manage the forums would have the power to reopen those threads.

It has been interesting reading and discussing a taboo topic!

BB