They want to finish 5th age quests before going back to 6th age... and the first few 6th age quests probably won't be a direct sequel to Endgame either. During the player polls, it was revealed that the Elder God storyline would include at least one experienced, two master, and two Grandmaster quests before the finale. We've seen none so far. Those plans may have changed in the last 3 years, but if they haven't, the experienced and master tier quests wouldn't require Endgame. So it'll be a while
To those cursed by war and pest, Come into the light of Armadyl and rest. This is the law of Armadyl.