i am not a top lane player but in my memory most of what you said is build dependent. ofc renekton is stronger in lane than out of it because of how his mechanics work with and around minions but he doesn't fall off nearly as heavily as some other champions.
If I recall correctly the new items like new BC, both hydras favor renekton heavily and basically opened up a bunch of build options for croc, which means he doesn't fall off as much as he used to back in the day.
all in all i don't think he's an late-game-oppressive mega scaling champion but he does work in many situations and can be useful in most games and also he has a very flexible range of builds that allow him to be very adaptive and do whatever the game requires.
you add to this that he does ok in lane VS a very broad range of common picks (the end of "age of renekton" was when Ryze and Lulu came to top lane and kicked all the melee duelists out) and you get a very under-rated solid pick.
you need a front liner that will get in the face of the other team and lead the assault? can't go wrong with croc. You need someone to jump on a carry and delete him? can't go wrong with croc. You need a front line dude that will give the initiator a hand and keep the DPS coming? croc's your croc. you need to bust some tanks? croc has a build for the job.