This about explains it for me. Although I didn't read much further than this. I started with an '06 and have ended with one. I owned and shot a bunch more up to 458WM and down to the .223. I settled on the 30-'06 because it does all that I want it to do and all I'd ever care to do. Most importantly because I'm cheap and so is the '06. You can find brass at any gun range, anneal it, clean it up and shoot it accurately. .30 cal bullets suitable for deer hunting can be found just about anywhere and so can a usable powder. I've loaded it up and down the spectrum from slow hard cast slugs for killing junk in the trapline to heavy hard hitting 200-220's and hot running 150's for stretching it's legs. Will smaller rounds work and do just as well? Yup, I've done just as well with the .223 and 50-55 gr slugs. Are we starting to use smaller diameter rounds as a whole. Depends on your definition of "starting" and "we". It's a good thing if "we" are. Nothing will ruin a new shooter/hunter quicker than heavy recoil and muzzle blast. One could do just fine with a well tuned .22 or 6mm for most all NA hunting near and far.
