There will always be a new jailbreak for devices, apple will never be able to stop it. It is just a matter of how long it takes to release a new JB.. some times very quickly and others a lot longer then expected
The part of your post that is bolded and underlined is technically incorrect. I can think of several ways to stop jailbreaking off the top of my head and I have been out of the game, so to speak, for at least a decade.
1) They could put the firmware in ROM and run the OS from a split memory system.
The side affect is, no more iOS updates and the OS would be slower because of how it would have to constantly switch from ROM to RAM to run the OS. Only attack vector is a piggy back CPU design.
2) Fully signed firmware.
The side affect is the OS would slow down by an average of 25% to 40% as the system constantly had to scan the signature and hashes across each section of code that is signed. Only attack vector is breaking the hardware that is checking the hash. E-Fused devices at present haven't been broken.
3) Custom one off encryption keys for each iOS device to the core functions that allows super user to exist. This is similar to #2 but removes the need for large signature/hash comparisons at all times.
The problem is Apple would have to deliver each update signed with the specific key for that device or never update the kernel again. Only attack vector is the same as #2.
4) Remove the docking station port and only have a head phone and a very simple phone charging system. IE remove the ability to tap into the phone without pulling the unit apart.
This one would simply create a new cottage industry in hacking cell phones. While it is still possible, it would add a cost that some people may not be willing to pay. Like a new housing to support the modification.
Those 4 ideas took me less than 90 seconds to come up with. And some of you reading this might ask, "Then why aren't cell phones better protected if you can think of solutions"? The answer is again; time, money and hardware. How much is Apple willing to spend to defeat Jailbreaking vs making the device easy for them and the user to maintain over time.
By the way, one last piece of information to chew on. Apple is only interested in security for one reason. To protect their intellectual property rights in the court of law. If they don't make some "effort" to protect their IP, then a judge has the ability say "Apple doesn't care, why should this court". Interesting huh?