This is quite an interesting thread with lots of similarly interesting opinions; The charts in the OP does show a progressively decreasing engagement in merit exchanges which could be indicative of a decrease in overall engagements, but previous higher numbers may not be something to miss as the forum has for long been filled with spammers leading to changes like the newbie jail and enhanced newbie requirements and the early period of the merit launch led to the growth of lots of account farms - Not all traffic is beneficial.
About users losing interest; This could be due to the retro forum layout which is a bit difficult to navigate for someone new to the forum, I can remember how alien this forum looked to me in my first few weeks here, but anyone who is truly interested in Bitcoin should be interested in this forum, regardless.
As the OP mentioned, there are lots of useful information here and members do not have to be active or even logged in to access them, if I didn't have much to contribute, I could simply use this forum as a digital library for Bitcoin and my presence would not be felt, I can imagine some use it this way.
Does the administration need to do anything to influence traffic?
No, in my opinion. Bitcoin does not change with the times and does not market itself, this forum is a reflection of Bitcoin and should mimick it.
Comparing bitcointalk to other forums is not necessary imo, as any other forum run the way this is would have lost almost all of their userbase, regular social platforms constantly market their service and make regular changes to their software, btt on the other hand has had the exact same outlook for over ten years with no updates, but still retains a good number of active users.
As I've mentioned in another thread, a new forum software would not matter much in attracting new users, but only to add changes that majority of members have been requesting.