Jax is as good as he was before the newer characters were released. Take the newer characters away, and jax is still completely viable.
All the new characters are just better in comparison to the others. Tristana and Ashe don't suck, Ezreal is just better.
Jax doesn't suck, Garen and Xin are just better.
The trend with the new characters is they almost all have an easy early game. Give a good player an easy early game and they will be able to take full advantage of it. The new characters are as good as or better than the older ones when fully geared, and get there way easier.