I think it will last quite a few years before they will even have to consider shutting down any servers.
Having said that, the game has lost a TON of players. Compared to how many people online around 2007, in the game's prime, it's weird to see only ~100k people online versus 250k+ back then. I think another problem is a lot of new players aren't interested and just don't come back.
It's truly become a pointless game to invest time and money into, in my perspective. Nearly anyone with a 2k+ total has most likely botted their stats. Staking and pking are infested with glitches and ddos attacks. The majority of players take part in dicing, which is one of the most unbelievably stupid fads runescape has ever had.
The game hit it's prime in 2007 with barrows armor and whips. Weapons like chaotic mauls, d claw specs, and the d bow spec absolutely ruined the game. You should NEVER be able to KO another maxed player with one hit on any spec.. PKing (especially) will turn into rushing, and that takes the LEAST skill.
Also, for me, I feel that Jagex lost its touch with new skills. I remember when runecrafting and slayer were added back in the day. Now THOSE were legit skills. Farming was decent, but totally useful. But seriously.. summoning? Dungeoneering?? CONSTRUCTION??? Not to take anything away from anyone who has 99's in these... I don't know how you do that without dying of boredom. What is a players' motivation to get a 99 in skills like construction? It is near impossible to profit in anyway from construction, and and there is very little reward for having a "nice" house.
When we trade accounts and sell RSGP for cash, we only make it worse. Just my opinion, it's already a ruined game though.