What we saw was the end result of 200 million more years of evolution. After a mass extinction - which we didn't see - there would be a transition period when the land and sea was, indeed, relatively empty, as animal species began to adjust and evolve to fill empty niches (such as the era just after the dinosaur extinction, when birds looked to become the dinos' heirs). By "empty" I don't really mean totally devoid of life. Say most fish became extinct, which would be even more profound an event than the extinction of the icthyosaurs and plesiosaurs. This would happen over a relatively brief period of time, geologically speaking, but the seas would still be teeming with life. It's just that some niches would be unfilled for a while; whales and dolphins took several million years to evolve to fill the icthyosaur and plesiosuar niches. Sharks, assuming they survived, would have to become better and better hunters as they pursued the dwindling fish population. Eventually the sharks would need to find new prey, but little in the seas besides fish were of the size necessary to sustain sharks. Cooperation would become important as large critters are tough to take down by individual sharks, and large groups of small creatures are easier to "corral" with help. Additionally, there would be a time when the only sizable prey in any kind of quantity would be other sharks (still holding to the premise of the show), thus making visual clues to pack association important.
I think you are correct that the show didn't do a good job of showing how the intelligence of the sharks manifested. The pack hunting diagram was not very good.