Raise the Tel Var drop rate. I don't know by how much. Perhaps by 100%. Taking all flags above ground is 200%, and boss farming there should still be the most lucractive. The sewers shouldn't become the main attraction, but attractive enough for more people to farm there.
Nothing about the sewers rewards you compared to when IC was new. The flag bosses are tedious to solo or extremely dangerous, but there is no reward for doing so. Trove scamps are only worth it, if chasing the achievement, since you can buy Hakeijo with Tel Var. Molag Bal is a complete loss most of the time. Portals are OK. Minor bosses, even if they're easy, take more time than ordinary mobs and should reward accordingly.
The reward structures were left untouched since before the flags were put into IC and are simply out of whack. You need change nothing about the design of IC, but if the sewers rewarded you appropriately for your time, I am convinced you would get more people to go there and spread out within IC.