yeah, as i said in the OP, often a merge is justified and makes obvious sense (“which reverb should i get?” getting merged into the reverb thread, for instance).
for those cases where it’s less obvious i think contacting before a merge would really benefit the forum. i’m very cognizant of the fact it could mean mods having to spend more time, but it might also save time if a merge has to be reverted. another point to consider - when somebody posts a new topic, they might save some thoughts for a later discussion, which could move it in a different direction (same applies to categories too), so a first post might not necessarily reflect the overall topic. a conversation would clear this as well.
more importantly though, not everybody is comfortable contacting the mods. which means (as is evidenced in this thread) that some people might not feel like posting or contributing to a discussion anymore. i think we all feel strongly about lines being one of the most welcoming online communities. i’d hate to think that some people would not feel that way, and instead of expressing it they would just withdraw quietly.