Well,I wouldn't say that Ashe is worst ad carry or not viable at all,her volley does less damage but it has slow.I find her quite dangerous to lane against,but I cant disagree about passive.
It got to be one of the worst passive's ever,I mean,yes,level 1 ,u can get your crit on the enemy champ,but after that its soo useless.
I mean,lategame your crit chance will be high enough as it is,passive will have no use at all.
Even while laning,you have to lasthit,you don't have the time to wait to get ur 100% crit.
IDK,maybe they should transform hawkshot's gold bonus into her passive(like TF's,but only for her,very useful on AD carry,since farm is the key) and,because of that, decrease cd on hawkshot/increase duration or something like that.