I do like to see when people try to look into some actual numbers and back up their thoughts with some reasoning as of why, always very good, so many pats for that.
But some other good points has been mentioned above, and domino effect is probably something that can happen too.
In general, I do not think that the way the registration system works should be the first aspect/scapegoat to get looks it's way when it comes to the question or claim "Is FeralHeart becoming inactive/dying?". If I will be honest this is something that has been fairly common to see people mention, ask or talk about even within the very first year FeralHeart were here. I'd bump into people talking about in the game, and maybe an occasional forum post or journal somewhere not too far into the lifespan of this game, it has gotten a bit more common to see "today" however but I do wish to point out it has been fairly present over the years, so even though it's up for discussion whether or not the user count is going down and in which ways, I personally feel like I cannot take these kind of talks too seriously in most cases.
Even though I can understand the importance of updates and patches, and how it has played a big role in user activity/count we have throughout the time, something related to this is in general what the game is and has been at it's current state, it's a fairly basic chatting/roleplaying game at this point at heart, and many will grow tired or even bored of it quickly, even if things go well they just need something to do and hosting roleplays or making maps all day long might not cover up everything a certain user looks for in a game, and once the "honeymoon" sort of attitude for the game passes, many loose interest quickly and move over to something else, but of course it doesn't help that the last patch the game got was in late 2011(close to new years if I recall correctly).
From what I have observed peak times are not as active at they used to be certain days let's say 2 years ago, but I do feel like the game is holding up well considering on what it is and what we have to offer. The lowest amount I clearly remember seeing online on the server(not counting in times of server restart or other issues then it would be 0 or 1 user online), is actually around 21 users, and this was back in 2011 just a few months after the game launched, even today at the most dried out times I still catch user counts above that.
Another important thing I want to point out to mostly aimed at the OP of this thread. It's very important not to confuse the numbers when it comes to "users daily" and "currently online" being displayed in the top left corner of the site. What it shows on the page is only the current amount of users, and it changes drastically throughout the day depending on what sort of time it is "where" around the world. One point of the day we could have 100 users online, another we can have 800, so try not to mix these up. Your 5pm could be someone elses 4 am, and it has clearly shown that american afternoon hours are those who tend to be awake and at their computer at server peak hours, for us in Europe timezones that means we'd find most activity past midnight. I believe "users daily" probably span over 1000 users easily.
So in general, there's a lot of things to take in here and consider and from what I can see, a lot of people need to take in consideration of how the world works when not only they are online and what "their" eyes can see. Hope this enlightened someone regarding this.