Lifesteal seems to be fine to me. ADCs are by no means supposed to be durable. They need to keep their builds based on damage so they can do their job, but be able to stay healthy somehow. This is where lifesteal is the solution.
By the way, if your argument is that having two lifesteal items provides too much sustain and makes the target unbeatable, know that there are many ways around it. Here's how.
Suppose a carry has a full build consisting of
. Doing a DPS duel would be a bad idea, since a good chunk of their damage will heal them, and they hit you hard as well.
One solution is to burst them down. Melt their health before they have a chance to act. If you can get them low enough, you will be able to kill them before they can start attacking. The shorter the fight, the better.
Another solution is to use crowd control. If you can stun them, you will get time to attack them while they can't lifesteal their health back. The more crowd control your team has, the more damage that can be prevented, and therefore, less healing can be done.
There are ways around mechanics like this. The limits are your know how and your skill.