I think it's not dead, and people are just ignorant when they say that it died years ago. If anything, the game that is saving the franchise is RS3 itself. Yes, OSRS may have more players (not by much - many are bots), but guess which games brings in the revenue?
Regardless of what people say, the game that'll keep getting more support, more updates, more attention, will be RS3.
Why?
Simple, first of all, it's the main game of the franchise. They've worked years to get to where we are, so they won't just abandon it. Second of all, during the year 2015, revenue from micro transactions alone was approximate $38 million dollars, thus increasing their profits to levels it hadn't reached in years. If anything, Jagex currently makes more money than it did back in 2007 when the game peaked.
So, to finish this off, many people think just because RS3 looks rather empty that the game is dying. Truth is, both games are somewhat similar in players. OSRS may have about 20% more at all times, but a big portion of those are bots. It's insane, bots everywhere.
If for some reason Jagex were to start losing revenue and the franchise itself started to die out, trust me when I say, OSRS will be the first to go. It generates almost no revenue yet they have to pay a developer team to maintain it. No, RS3 is not dead, RS3 is what is paying for OSRS and everything else.