Seems like the "old guard" has massive bias based on anecdotal evidence, but not modern science. Assuming good shot placement, if there is sufficient energy for the animal, velocity to expand the particular bullet, and quality bullet that can transfer energy, then the rest is academic. But that is with all things perfect.
What guides and others are really saying when they say a caliber is insufficient, in my opinion, is that larger calibers with more raw energy and better trajectories have a greater capacity to kill in less optimal situations (such as shot misplacement, bullet failure, etc. ,etc.). And they are likely right, that in less than perfect scenarios the larger calibers may get the job done when a lesser calibers without perfect execution will not do the job. It is all tolerance. If a 6.5 CM require a perfect shot (body angle, placement, energy, etc.) when a 300 RUM requires simply a poor hit to kill an animal, a guide will state the obvious - use the 300 RUM. With all that said, a 6.5 CM undoubtedly can kill big game animals. You just have to know the caliber, and your probability of making a quality shot given the variables.