I seriously don't understand Rengar's rework at all...

What did it do? Nothing. The goal, Riot, that you stated, was to make assassins get in and get out while doing their damage. Well guess what! That is the complete opposite of what Rengar does. Not only did you overload his W even more, he doesn't even build damage and can dish out a pretty hefty amount of damage. He is essentially a diver now, not an assassin. He is like Vi, except with a way too overloaded kit.