My brother has been playing for years and has a few paid accounts. Here's how he explained it to me. All paid accounts had their prices locked in until you cancelled them. His first, and main, account had a price of $5 a month because he first bought it 15 years ago.

There are also "ironman" modes that exist in the main game. It's an option at character creation that will restrict your account from trading with other players forcing you to obtain all items on your own instead of just buying them from the trade board. Since you need to make a new character, this is also another payment. My brother has two ironman accounts.

There are "leagues" which are new temporary servers where the rules are different and XP gain is incredibly fast. You're given tasks to complete before the "league" ends and are awarded cosmetic items based on how much you complete. This requires its own paid account to play. My brother has one of these too.

In total he spent about $20 a month on the game for his various accounts. This change to the subscription will set every single one of his subscriptions to $14 a month raising his monthly payment to something like $56 a month which is ridiculous. He plans on ending all of his subscriptions since there is now no incentive to stay subscribed (the price is no longer locked in). So my brother, a long time and devoted customer, will play the game less and give less money because Jagex is hoping most people like him won't go through the hassle of unsubscribing.

He, and lots of other long time players, are hoping that Jagex does what other MMOs do and allow multiple accounts for one subscription price.