Most answers here claim Obi Wan did not want to kill Anakin so he warned him about not jumping. I believe the opposite. Let's look at the fight in my low quality screenshots.
In the first and third scenes, Obi Wan clearly has the high ground, but it does not magically grant you the victory. Similarly, in the second one, Anakin easily relinquishes his high ground to pursue Obi Wan. A more careful eye might see even more ground difference during the fight.
Even more, Obi Wan actually tried to outmaneuver Anakin in the lava fall. His strategy was obviously let him take the fall and burn. Even at that scene Anakin fell to the trap and grabbed the other cable and chased Obi Wan.
Obi wan, clearly understood he can manipulate Anakin came up with this strategy. Let me break down the choice of words.
It's over Anakin: Playing his ego.
I have the high ground: "If you are as powerful as you claim, get even a higher ground."
You underestimate my power: Proof that it works. Now, as the question clearly mentions, he could have run or walk the climb and be in a relatively bad position. We all know that even with today's technology higher ground is quite important, and I suppose it might matter in Jedi fight, but still he won't get his limbs chopped off instantly. Or he could simply not risk it and run back to the first clonetrooper division and kill Obi Wan with their support. But he feels that he has to prove his powers to his former master.
Don't try it: Still playing for Anakin's ego. These words pave the easiest way to play with a drunk and Anakin was drunk with his powers.
Long story short, Obi Wan, realizing that if the fight goes longer he would be in a disadvantage, goaded Anakin to jump over him. That's why Anakin did the most useless jump in Star Wars universe.
A final addendum to the topic. When Yoda told him "The boy you trained, gone he is, consumed by Darth Vader." Obi Wan did not protest any further. He knew his task and he reluctantly accepted. Killing Anakin was his only intention in this fight and he tricked Anakin into a trap to do so.