First, Cold Steel doesn't even give you a full extra second of life unless you're being attacked for 7 seconds-- which you wouldn't survive anyway with the amount of armor penetration and/or magic and true damage available to on-hit fighters even despite it only being on armor items.
Second, one of the most abused on-hit fighters is _immune_ to cripples. Granted, he's also the one most capable of killing you well within 7 seconds so it makes little difference as Cold Steel is now.
It should be made an active effect so the cooldown can be used as a balancing tool, and buffed up to anywhere between 30% total AS and 50% total AS. That, and Master Yi should only be immune to movement slows during his ultimate, not cripples as well-- which makes no sense for balance on a champion whose passive grants an extra auto attack in the first place.
Perhaps this would need to be put on an item without armor, so it's not just the catch-all against on-hit builds.
Please note that introducing more situational counter build items against particular classes allows them room to buff those classes as well. (For example, Zed's allowed to stay incredibly powerful solely because of the existence of Zhonya's, which can halt his snowball early.)
Also, for anyone wondering:
15% of total is around 1/7th (I rounded). So at any attack speed above 1, you would only get 1 extra second of life for every 7 seconds you're being attacked. Keeping in mind that at 2 and above, you would need to be surviving more attacks, but you would still only get an extra one second.