As typical on this site, many people offer their opinion on a topic that is not based on first hand actual experience.
I put 2 Kayaks in my suburban, one 10’ perception and one 10’6" pelican.
The 10’ sits at an angle while the pelican sets on top of but its nose goes between the front seats.
No kayak sitting right behind your head ready to launch like a missile. The bottom one is secure btwn from seat and lift gate
You need a better understanding of simple Newtonian physics. That boat doesn’t “build” velocity; it already has the same velocity that the rest of the vehicle (and everything else in it) does. This is what seatbelts are for: to keep the passenger from flying out through the windshield in a case of violent deceleration. Think about tiedowns.
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No, a 70 pound kayak with the surface area creating friction and sitting pointed at a seat back with no room to move much isn’t going to get away from you without help. It would be wedged in and not going anywhere. At least as long as you make sure it is not able to move around much. The concept of it is already in motion is partial science. It is stable relative to the truck. It has surface friction. Movement has to be initiated between it and the truck. It would only have inches to move before hitting something. It’s already in movement is a false analogy. So is the truck. It’s the change in momentum between them that is the movement. We are on a spinning globe, but the concept of sitting still is relative to what we are sitting on. Difference in velocity up to impact with the seat is the velocity used to calculate energy, not velocity relative to the pavement.
I carried s 12’ in our Suburban. And lost the windshield.
F=ma. Same principle as in concussions… Deceleration injury. A big enough m with a big enough a is substantial.
In my 4-Runner the bow of the boat rests on the armrest and is trapped between the front seats. It’s rock solid. Some years ago I picked up a 16 foot canoe in Canada and it rode the same way in an Expedition for 100 miles and again was rock solid and the boat didn’t even have any wind pushing it around. Just bring your boat when you go SUV shopping and do a test fit. 
I had hesitated to weigh in on this one until someone above mentioned they had lost a windshield that way.
I started carrying my WW boat inside the car for pool sessions, an old Innazone 220 which is just workable in volume for that purpose. But one thing I never did is trust the front seats or console between to hold it in case of a sudden stop, because since better seat belts and all those air bags they simply do not make the seats themselves strong against an impact. Seat backs only stay put with the aid of the seat belt. And with just me driving and seat belts that can’t be locked into a rigid position I can’t use the belt to protect the passenger seat. (I frankly still miss the days of belts that could be locked into a position, I found it helped support my back on long days in a car.)
So any time I have put the little Innazone into my compact SUV, I also take a strap or rope and secure it thru the front carry handle and run it back to something on each side behind that. I admit that if I can only find the handle for the rear doors it is not great, better if I can find a metal anchor of some kind. I have done this in two compact SUVs of two different makes so what I am hooking thru has varied.
It is likely not foolproof. But it is something more than the seat back if something happens. Obviously air bags would deploy in a major crash, but air gas are designed to stop people who are buckled into a seat. Not boats.