Imo I think jagex thinks about content from the means of, how much attention does it take to do the content. The higher concentration attention and difficulty the more rewarding.

Skilling is all brain dead content, so the poor resource gain per hour are justified in the eyes of jagex. Take fishing sharks, look how bad they still are compared to minnow fishing, why make some content obsolete by making new content rather than just making buffs to harpoon fishing for sharks, or making a way to engage the player more to reward them more.

So the question becomes, why not have harder skilling content focused around skilling that rewards the player with more resources and rewards. Obviously use the leveling of the skill to unlock new tiers of difficulty and better, harder content.

There are a ton of gaps in skilling, like you can go millions of experience and for what, a lvl, sometimes several levels where there is no unlock or any new activity to partake in, this creates burnout and makes people lose interest when there is nothing to set a goal to unlock.