I've always seen this kind of posts in every MMO that I played with the eventual "THE GAME IS DEAD/DYING!!!" drama, my opinion is that people come and go and ofc you are not going to get as many players as when the game was still "young", people will eventually come back when there is a big update, grind what's new and leave again and so on, like... in any other MMO.
Lets add to the reason that MMO's are no longer the main lure for people as in for example the year 2000 to 2010ish, that Wurm just attracts a certain kind of gamers which is not the vast majority (lets face it, even although I love Wurm graphics, a lot of people will never get into Wurm because the """graphics are ugly / dated""", notice the extra commas...) and the release of Wurm Unlimited (and this is again my humble opinion) probably did not help to the WO community either. Although I know some people have returned from Unlimited to Online.
Most people that try Wurm don't get the point of the game and some reviewers don't help either with their harsh thoughts (recent example).
Also I've never ever seen any adverts in any shape of form of Wurm Online, I know this is not a Blizzard game but still... I came across this game years ago browsing Google when I was looking for a Linux MMO, and tbh the site didn't lure me much so I did not try it. Finally 6 months ago I came across Wurm Unlimited on Steam, we gave a look at it and decided to try the Online first (cause it's free) before getting Unlimited and we just got stuck on WO pretty much.
So, yeah... you still get new people. I'm a living proof of that lol.
As always sorry for the long post/rant and my grammar.