Why is league only balanced for the top tier of players?

I had originally sent a support request with the below text pasted. The response given was that I should post this on the boards. Unsure if developers/designers would see this, but I feel my points have some merit or at least deserve to be addressed. Below was my original text in the support ticket. I just played a normals as riven, laning against a nasus. During the early laning phase, I played well against him: harassing him, outfarming him, and even getting two early kills. Despite this, Nasus bought a single item early on, Frozen Heart, and from there on I couldn't do anything. I couldn't harass him because his armor was too high and he wouldn't take enough damage to deter him from farming. I couldn't outfarm him because he was no longer scared to farm. I couldn't kill him because his armor was too high for my damage at this or any other point during the game. His lifesteal made sure that any good trades I did get on him, he would right back up from. At this point, the game was all but lost. Despite doing well against him in laning phase and making sure he was very far behind, all it took was one item for him to instantly be back. And I don't mean back in the sense of "oh he can do some decent damage and is something we should watch out for", I mean back in the sense of "oh he is unkillable and will dive turrets for 30 whole seconds and two shot us and there isn't anything we can do about it". I find it absolutely ridiculous that not just me, but my entire team could do anything about it. However, I know that isn't something you're likely to change, since Nasus is terrible in higher tiers and pro play. My question to you is multidimensional, starting with; why is this sort of situation allowed to happen? Why can a player play well and do everything they're supposed to do and still fail because one champion with one item can snowball the rest of the game? Secondly, why is the game only balanced for the highest tiers of players? If you are trying to bring in new players (which is seems you are from all the new player centered commercials I've seen) wouldn't it make more sense to balance the game with this new players in mind? Thirdly, are you planning to rework nasus? His design has seemed to consistently be overpowered at lower tiers and useless at higher tiers and this seems like it would be called for a rework or maybe a quick fix. Thank you for taking the time to read my request. Sincerely, a frustrated low elo player