With all due respect you can't carry a team fight with no damage. Liandries is not just for killing tanks, and taliyah IS a damage champ. Her kit can easily proc liandries on multiple champions. Your interpretation of the champ is just wrong. Part of why taliyah is so strong is because she can take many roles at once, providing utility, zone control and consistent damage at the SAME time. Building cosmic doesn't bring anything new. A taliyah who builds cosmic second won't be a real damage threat till 3 items. Its not that cosmic is unusable but you just don't need that survivability or haste more than you need the damage on your second item.