Jagex keeps teams dedicated to the game
I do think the activity spike in RS3 is attributed to the hype and integration of Legacy mode to some extent. Those that disliked EoC mechanics might have left the game nearly 2 years ago... and thought they were through with it until a dear friend told them about the turning back of clocks. Before OSRS was released, there was also a surge of popularity in the live game version. When it was introduced, many players tried their hand at oldschool and either stuck with it or disbanded from the game once again. As seen in the graph above, the trend for OSRS throughout 2013 is essentially a steady decrease. Looking back, I'm not sure why the live game followed the same trend. Major updates included the Order of Ascension dungeon and new skill Divination, which rejuvenated some interest before gradually decreasing again. Maybe it was the constant Squeal of Fortune promotions shoved in faces or the lack of significant game-changing updates, whatever it is, it affected the popularity of the game. A spike of activity at the beginning of 2014 could be linked to a new year - one "promised" to put emphasis on players with an in-game poll system, skilling updates, and whispers about Elf City or Invention. Now if you closely examine the numbers in July 2013 and July 2014, you can see there was more players online during the previous year. You are correct in saying that activity during last year's summer overall is lower than this year's though.
Honestly, I don't believe the live game will die, provided that Jagex keeps teams dedicated to the game... and continues to hire creative people who are passionate about designing fresh content in a massive game. If that ceases to exist, then you can guess what will happen. For as long as the live game progresses, while keeping some of its former glory, interest will be continuously gained. It's pretty difficult to tell if new players are taking interest in the game, but it's the combination of new people and a loyal player-base. I also don't think the increase in popularity this year is due to bots. Introduction of Botwatch, right-click reporting, automated ban systems, and successful integration of bonds has almost eliminated the macro population completely. I'm sure if you ask anyone in the live game about this matter, they will say it's pretty hard to find bots that won't be destroyed within the day. Of course, this comes with limitations and mistakes. As a CL of the Questions board, there's been an increase in the number of topics on wrongful bans this summer. Regardless of whether they tell the truth, a system is only as accurate as its programmer. It is not human. That said, there has always been and will be mistakes on their end. I just think the wrongfully banned have only recently decided to post on Zybez. Maybe there is something in their engine that even they don't know about, either way, only the ones accused of such activity have the answer.
I dare say that 4rsgold is the most popular forum space for Runescape because the sheer number of discussions on both in-game and off topic issues.