^People have been saying that RS will die some day for over 10 years now already. While it is logical that all things will die in the end, it doesn't mean that it will die before the company itself does. Also while RS can die, it is also probable that it will blossom more.
On the topic itself, this will be a game that I won't even try. As I don't like FPS at all (as I am a pacifist I just don't see the fun of shooting others, I find it boring).
That's just not how modern corporations/companies work. They will try to exist as long as it is possible, with or without RS. It's more than likely that RS will die before Jagex does.
As for the probability that Runescape will blossom more, I don't see how. Runescape is a very old game in its late phase. It hasn't blossomed for years. Its player base has stagnated and declined drastically for years. The game operates basically on a freemium model (unlike the past when it operated on a subscription model). Basically, it operates by micro transaction payments (SGS/ TH) - it extracts a lot of profit from a very small segment of players who are very hardcore and (relatively) wealthy in real life. In freemium models, a very very small minority of the player base generates most of the profits (often less than 1%).
It no longer uses a subscription model as its primary means of profit (though obviously they still do sell subscriptions, but they're a smaller part of Jagex's profits), that means they don't depend on constantly having newer players sign up, and then convincing them to buy subscriptions. The numbers bear this out: free to play is practically deserted and Runescape no longer drawing newer players to the game like it used to. This leads to a contradiction where even though Runescape is perhaps more profitable than most of its years, it has less customers than in the past ( a pretty small part of the customer base that actually pays most of the bills - those who buy bonds/SGS). Well, these players will move on at some point in their life, and when they do, it will be over.
That's the problem with trying to run a huge company on the backs of a small segment of the customer base: if they leave, it could all crumble. With a larger customer base, you aren't so dependent on so few people, and your risk is dispersed across a large number of customers. After all, in the past when Jagex had 1 million subscribers with more cycling in each month, it was very unlikely that they'd all simultaneously quit. But now that they depend on a few hundred or thousand people at most to buy bonds/spins/SGS hardcore, well it's not very unlikely at all that a few hundred/thousand people decide to quit the game.