her win rate is high but it isnt astronomically high. astronomically high would be like 58-60% (ps you havent given a solid reason why janna is so unbalanced). honestly i wont say she doesnt need any work but she certainly isnt near as op as the boards claims she is. jannas win rate is almost certainly so high because she fits perfectly in with the meta right now because right now everything is about adc's and shes very focused around the adc whereas most other supports are more for the general team. essentially that and her e's compatability with ardent censor is what makes her so dang strong right now.
anyways, i still believe that they should have nerfed the item because of the sheer number of champions that could abuse their power, but we'll probably just have to agree to disagree there. my problem with that more lies in what i said earlier, that they have a bad habbit of nerfing A in compensation for the strength of B, then nerfing B without rebuffing A to compensate, leaving it in the dumpster for a long while.
thats more or less what i was saying, but i would say use it to determine which champ COULD be overpowered, because i think we can all agree it isnt always the champion that gives them the high win rate. i entirely disagree on your next point, the only case where win/pick/banrate alone can determine if a champion is breaking the game is if theyre ASTRONOMICALLY high, for example if i recall correctly at one point kassadin had something like an 80% ban rate, and at one point pantheon had like a 65% or something winrate. thats a clear sign that the champion needs an immediate nerf. however, most of the time those statistics alone arent enough to determine whether or not someone needs buffs/nerfs. i wont pretend like they should be ignored, but they certainly shouldnt be taken at face value as gospel. i still dont think janna is overpowered because after playing with as and against her and looking at the current meta and other issues in the game such as items it still seems to me that the problem is primarily not with her own power.