We can adjust for inflation, sure, but we also have to keep in mind a growing investment from Jagex themselves, right? The OSRS team literally quintupled in size over the past 8 years, no? The game is far more expensive to run today than it ever has been.
That isn't to say the membership price can or should increase indefinitely, but it's a relatively small price to pay for a game that is unpolluted by the kinds of monetization practices every other popular MMO on earth has.