Jax is perfectly viable atm and is even a strong pick if you know how to play him.
Except for sustain, Jax has all the tools he needs to beat every opponent he can face in top lane if you play him RIGHT : high base stats, high base damages and scaling, low CD on all his skills, hybrid damages, gap closer etc.
Jax's early game is weak only because he is gated by his mana (even in early game Jax has already big damages) and the fact that he has to commit himself in early game to last-hit.
But you can perfectly play around his weaknesses by using the ward trick (upgrading your yellow trinket as soon as you can is really really helpful), by knowing how to freeze a lane / create slow push etc.
Even Gnar and Olaf (they are far more deadly to Jax than Renekton imo) are manageable if you know that. I'm not saying that doing all of the above is easy, in fact, it is quite hard and it will require to play Jax A LOT and until then, you will have several very painful games. But Jax has quite a big learning curve despite being "straightforward".
Hell, even Fiora is perfectly manageable until very very lategame.
Also, he has insane follow-up in case of ganks, even in early game, thanks to his high sticking power and stun. Ganking a Jax is never a bad option.
PS : Jax rapes
hard. The MU is the opposite of what it was before the rework. Jax's Q is super fast, making it easy to dodge the full damages of Decimate. Darius also has no longer an AS debuff, his own base AS is now also bad. All of this, combined with the fact that Jax's E fully dodge Darius' W and AA, makes the MU super hard for Darius. New Darius has big trouble to stack his passive against Jax. And without his passive, he is an easy kill for Jax.