@Tesh Valura
"If gaining XP is no longer the main purpose of this game, what is?"
Entertainment and fun, a vast variety of them. For instance, exploration, stories, achievements, competitions, visual/audio features, etc.
"If the skills are too slow for a modern era MMO, why are Jagex's numbers not suddenly indicating that DXP all the time is the only option?"
Because they have other ways to increase the xp rate, e.g. Daily Challenges and free xp, live events like Postage Pete etc. And don't forget there is no saying Jagex won't further step up their DXP Live efforts in the future to 5 or 6 DXP events next year.
"To say that gaining XP at a doubled rate every three months is the only way to survive in a modern era is very, very incorrect.
You don't draw in longtime players by handing them end-game content."
You read it wrong rather. I mean gaining xp is not the main focus for the survival of MMORPGs in the modern era, the ENTERTAINMENT, not necessarily end-game content is.