The orca with cotoyé the mégalodon during thousands of years. Result??? The orca continues to make live its species, and the mégalodon not.
The Orca has co-existed with Carcharodon Carcharias for millions of years and these two animals often compete for food in same regions. Result? They both are still thriving.
So do you think that your logic makes any sense?
Therefore, the orca showed itself quite higher than Mégalodon.
Wrong conclusion!
Orcas came in to existence about 5 million years ago in the Pliocene Epoch but still Megalodon had remained at the top of the food chain during the entire Pliocene Epoch, which ended about 1.8 million years ago.
Megalodon was actually specialized in hunting large animals including "Whales (big or small)." It was a perfect killing machine and dominated the marine world for millions of years.
But nothing lasts forever as evident from history and this concept applies to any living being (including us "humans") as well.
PS: at the predatory large level, I do not know if the mégalodon would have made the weight vis-a-vis in famous Liopleurodon which had some 3 meters length jaws and teeth of 40 cm... Moreover, it measured 25 meters.
The skull of the largest known true Liopleurodon individual is about 1.5 m long and this creature is estimated to be about 11 m long and weighed about 5 tons. This animal is actually a "Liopleurodon Ferox."
That 3 meter long skull that you have mentioned here actually belongs to a creature which is termed as “Monster of Aramberri.” But this animal is actually not a "Liopleurodon Ferox" and has been wrongly and pre-maturely portrayed by media to be a Liopleurodon. This newly discovered Pliosaur is estimated to be about 15 m - 16 m long.
However the remains of this Pliosaur has revealed that an even bigger Pliosaur had existed and it attacked this creature and that monster probably had a 4 meter long skull and could have been about 21 m long.
Otherwise, since according to last the scientific studies, the mégalodon measured 15 meters maximum
Wrong again.
The maximum size of Megalodon was calculated to be about 16 m but this calculation is based on a 6.61 inch Meg tooth. And this estimate is now obsolete because people have found several bigger Meg teeth and some of these teeth are over 7 inches long.
So the latest findings indicate that the Megalodon could reach 60 feet (18 m) in length or possibly even exceed it.
I think that a 10 meters length male orca, could have killed it while sinking in the belly of the shark.
You forgot to consider several other factors here.
1) Size:
An adult Megalodon is far bigger then an adult Orca. (18 m vs 10 m)
2) Weight:
An adult Megalodon is 6 to 7 times more heavy then an adult T-Rex called “Sue.”
And some paleontologists have estimated that this shark could have weighed even more.
3) Power:
This monster shark was specialized in hunting large animals and it had to fight with various species of Whales on regular basis in its life and who would come out victorious in such struggles is already clear to us through fossilized evidences.
4) Damage:
This shark can chomp an adult Orca in to two pieces in a single go.
5) Psychology:
Orca seldom confronts the Bull Sperm Whale because it is dangerous.
Now a Megalodon is even more dangerous because it is more agile and will fight back more effectively.
Also small predators try to avoid confrontation with predators that are bigger, more powerful and dangerous because such encounters can be fatal for smaller predators.