Different outlook on the Jedi from OT - PT

When the OT came out originally my impression was that the Jedi were these mystical warriors who had abilities far above others.

This is overall true, but as Obi-Wan said in ROTJ: he took it upon myself to train Anakin and he was wrong.

So they are fallible and able to make errors in judgement...

Qui Gon takes on Maul and loses his life, Anakin takes on Dooku and loses the best part of an arm, Luke takes on Vader and loses a hand.

So they can err in combat and suffer losses.

The Emperor(being the one who engineered the end of the Jedi Order) knows this and yet it would be not acceptable to him for a Jedi to spring up again and potentially upset his best laid plans.

Apart from replacing a weakening Vader,why keep Luke alive as a threat??

If the Emperor feared Luke in any way, then there's a suggestion of a certain amount of power in Luke that the Emperor wanted terminated(if he couldn't control that power).

 

Yeah, and don't forget the TPM line:

Anakin: "I don't think so. NOONE can kill a Jedi."

Qui-gon: "I wish that were so."