Runescape was a cultural phenomenon in the mid to late 2000s. The Free to Play MMORPG offered a credible alternative to those who could not afford, or didn't have a good enough PC, to run World of Warcraft and gave players a huge world to explore and many skills to level up.
Runescape continued to be supported, with graphics updates and including quality of life improvements such as The Grand Exchange in 2007. This couldn't stop the inevitable drop in players when the MMORPG bubble started to deflate in the earlier part of this decade.
Jagex, ingeniously, decided then to go to the community, to see what they wanted. This is what leads them to release "Old School Runescape" in 2013. This version of the game ran separately to the modern version and presented itself as a stripped-down, back to basics approach to the game, without the graphics and gameplay improvements. Soon people returned looking to recapture their youth and return to the world of Gielinor.
Jagex ensured the survival of Runescape. To this day Old School Runescape alone still has over 100 active servers facilitating up to 2000 players. This move was so brilliant, MMORPG giants Blizzard emulated it in 2019 with the release of WoW: Classic.