He has way more tools than any other assassins have. There's no way he needs all of those things when others don't.
Just his ult has a leap, stealth, movement speed towards enemies, gives vision, combat MS AND ferocity generation. What other assassin has or needs anywhere NEAR that?
Comparing removing ONE of those mechanics to removing an entire ultimate is just a stupid argument. He's tanky, has sustained damage, has burst, has steroids, has heal, has sticking power, has cc, has gapclosing, has escape. He's got all the strengths of an Assassin and a Diver, but none of the weaknesses. If any other champion can be balanced and have their numbers based on less than a million mechanics, I think it's a fair guess that the same could be done for Rengar.
I don't care in particular about how strong he is at the moment, he's just overloaded, poorly designed and lacks counterplay. The game is at it's worst when he's strong, though.
Only those that can't be arsed to have any weaknesses or counterplay likes him, which is why they throw a tantrum every time someone tries to actually design him like any other champion.
The ONE THING of substance that's unique about Rengar is his bush-leaping, which is entirely overshadowed by the fact that his ult lets him do it anywhere. So much for situational strengths.
I'd ask you if you have any intelligent counter-arguments, but unless you have anything more thoughtful than "git gud or git rekt" I don't think you should waste your breath.