No one (at least if they understand what they are doing) is really building a sheen item (be it IBG or TF) for the damage enhancement. I think that idea went out the window years back. They are gotten for the utility they bring, which I think you may be vastly underrating.
I wouldn't agree that IBG is only a fallback for when you are behind and need to go "support" style. It gives you insane sticking power in skirmishes and gives you a markedly improved mid-game with the slow proc. It allows you to wither a target, catch them, proc the slow on them, and then wither again, borderline perma-slowing people. While alone SV and IBG are only 30% CDR, those 2 items + boots gives you a FAR stronger mid-game than most any other Nasus build in the history of the game could offer.
Tforce is also vastly underrated in its current iteration and I don't doubt that we will be seeing nerfs to it. The rage passive is just beautiful for Nasus. SV/FH + tforce + HP item is a true hyper carry build for him. I haven't been able to play much at all lately and haven't gotten to really play test it, but I don't see how it could NOT be good, as it has great synergy with Nasus and is possibly one of the strongest items in the game ATM.
I am by no means arguing against LW, as it absolutely is the most cost-efficient damage boosting item you can pick up on Nasus if you have good Q farm. I'm saying that looking at IBG/Tforce as damage items is a misnomer and discredits the potency they bring with their passives, both of which are truly amazing on Nasus.