I'm a relative new-comer to bow hunting. I started shooting a recurve around 1956 or so and then quit shooting bows entirely when I graduated high school until a few years ago.
I have been shooting a rifle quite a bit over my 73 years, though, and some of the same arguments apply. I've had friends who hunted elk every year with a .270 and nearly always scored . They killed elk very dead at 200 yards with that little pea sized bullet. I've also read lots of articles written by guys who have made a lifetime of hunting/guiding elk and most of them say the same thing - they like a .338 over the smaller calibers for elk because:
a. Elk are very tough animals.
b. Most importantly, many hunters who have perhaps a once in a lifetime opportunity at an elk will want to take any reasonable shot. If they passed on every shot except a close-up, broadside shot and could place the bullet right where they wanted it, most anything .25 caliber or up with the proper bullet would do fine. It's the quartering shot that would be a piece of cake with a larger caliber that could result in a wounded, non-recovered elk with the pea-shooter.
I'm hearing the same arguments in this thread and I think they are valid. Those who say a 40# bow will kill an elk just fine will attach a few extra caveats beyond the usual "well tuned bow/arrow combo", "sharp BH" and "accurate shooting".
If you live where elk hunting is a routine activity, you might pass on a shot that others would find very difficult to turn down. I think if I was planning an elk hunt, even at my age I would work out to where I felt comfortable with something with a bit more steam behind it. For the time being, I'm sufficiently challenged trying to get an arrow into one of our mostly nocturnal whitetails this year.