Before GE we had falador park/bank marketplace. if anything, then GE helped the situation, because now people don't have to stand in the marketplace to sell items. they can just load it into the GE and go do something else. But having an active market is something emps world/emps scape have had for as long as i can remember. the reason why you aren't seeing people playing is because they either have completed their goals and waiting for new content, trying to merch, just want to talk to people or they simply don't know what else to do.
Most of the content above level 90+ in each skill doesn't really offer much ingame rewards, and only provide some kind of bragging rights.
we don't have to be concerned about GE. the game just need more gameplay oriented content, and more players.
Something like dungeons with increasing difficulities and rewards would be very nice. that could also open up for dungeoneering skill.