The original statement put forward was that kinetic energy doesn't matter. Clearly a false statement, and a foolish one. It does matter. That reduces the question to, how much kinetic energy is sufficient to guarantee enough tissue damage... and can the use of magnum cartridges supply this energy through velocity at extended range.
Can a 6 Creed kill a deer, or even an elk, at 500 yards? Surely. At 800 yards? Deer, maybe... elk, not responsibly/ethically. What about a 240 Weatherby Mag, or a 6 ARC? Of course, a .243 Win with a 1:7" twist can do anything a 6 Creed can do, except be bought over the counter. It's about retained energy, as you acknowledge with your remark about 40 gr .22 bullets. What is the range limit? That at which the bullet lacks the velocity to create the energy required for expansion.
Bullets need a minimum velocity to be effective, because they need a minimum kinetic energy to be effective. The push to boost velocities was the objective between WWI and the 2000s to solve this problem... an inefficient brute force approach. Now, the emphasis is on retaining velocity with VLD bullets to increase efficiency. Keeping more of what you started with works, to a certain distance. Do you need a magnum cartridge? Past a certain range, even with VLDs, yes.
Re elk at 500 yards plus with a 77 gr .223 bullet, it's important to talk about the failures, not just the successes, to get a valid understanding. We both know that if there are failures with conventional hunting cartridges and bullets at closer distances, there are failures with the .223 at extended ranges. I've seen coyotes smacked a little far back with a .223 at 200 yards get away (hundreds of yards and over a ridge with a gut shot). Are deer and elk less resilient than coyotes? Using a purpose-built .223 for long range big game hunting is a stunt, an exercise in ego gratification over assurance of an ethical kill.
The world record brown bear was shot and killed with a .22 LR. Is that a good choice for hunting brown bear? Just because you can, possibly, do something doesn't make it a good idea.