To begin, this "question" reflects a depth of ignorance that makes the Grand Canyon look like a road side ditch.
No, Mr. Ackley did not "improve" everything he could get his hands on. In fact, in my IGNORANT youth, I asked Mr. A about improving the .308 WCF and he advised against it. Said too little to be gained, if anything. In his writings he did not recommend improving the .30/'06. Only with the heaviest bullets and slowest powders was anything gained and then not much... So the statement begins by smacking of silly ignorance from the beginning. (Today there is a wider selection of "slow powders"
He retired around 1985.)
For the student who studies... Mr. Ackely was young when the "smokeless era" was still fairly new. Most grand dads still clung to their Winchester lever actions and black powder (loads were available until the Depression/1930s). The 1886 was available in .45/70 Gov. The 71 just came out in .348 WCF! While Junior was back from the war and lusting for a surplus Springfield '03 bolt action. ($18.00 thru DCM via REA to your door, or was it $18.50. Gun was $12.00 with a real walnut stock. The rest was shipping and handling). .270 WCF came out in approx. '26 to use the brass ('06) that could be picked up free in many places. Shove it into a die and out pops a .270 that doesn't kick as much... and away went Jack O'Connor on his literary/hunting career (when not teaching college)...
Many affordable cartridges were shaped like funnels. .303 Brit. 7x57 Mauser. .30/40 Krag. .33 WCF. .25/35 WCF. .38/40 & .44/40. If you read his books/remarks --he did a column in a number of gun magazines for years. Very helpful to the young and ignorant like I was-- he points out that by straightening the body of these cases you can keep more of the powder in the case as it burns and not allow it to be funneled into the barrel to burn there and accellerate barrel wear. One of the reasons for the steeper shoulders was to direct the burning powder granules into the neck of the case and not let them hit the throat... Obviously, the case is replaced/replaceable... throat wear much more permanent.
Yes, then there is the additional capacity. Mr. A was not alone in the "improved" activity. Mr. Epps for one did the .303 in Canada much. Offspring still operate the shop and offer the services. (link fromn 303british.com) You could have the SMLE in a smaller caliber, factory case necked down or improved OR necked up to as much as .338... And with a P14... .303 Canadian Mag rivalled the Weatherby... I believe the Weatherby line was developed in the M17 rifles and based on much research...
If you buy his books and read... you will see dozens of names. Mashburn. Juenke. Titus. Powley. Barnes. Howell. Gibbs. Many were more radical than Mr. A. Fred Barnes had a .224 QT (quick twist) that used a 125 grain bullet. Obviously, he made his own bullets. All sought what was not available readily and affordably from the factory. An old, dear friend, retired gunsmith told me that after WW I Remington used the P14/M17 actions (barrelled actions???) for sporting rifles. Model 30 maybe. Later 720? And finding one in .30/'06 for someone with the money was doable. Finding the .257 Roberts that were listed in the catalogue... He almost never saw or heard of one. Of course, Remington would not think of chambering the .270 Winchester... (ha, ha)...
In the no liability/low liability climate, machine work in a basement permitted many to buy surplus Mausers and turn them into fine sporters and sell them at a profit. FFl's were $2.00. Running a reamer into a 7x57 chamber to "improve it" meant you could rival the .270 (which 'sucked' until slow powders came out after WW II).
Mr. A also wrote of reducing the load on the action with the "improved" chamber. Loads that would "lock up" a Savage 99 could be much exceeded in an improved ?? .250 Savage? .22 Hi-Power? For the woodsman concerned with reliability... and the extra preformance was "frosting on the cake." Claims he fired a 94 Win in .30/30 IMP without the locking lug in the action... no problem.
So the cowboy who could not afford a new .300 Savage could come close with a rechambering of his 94 (or Marlin) in .30/30. Mr. DeHasse writes of all the people who condemned him for "butchering" Winchester single shots into modern cartridges. He points out that at that time, with velvet lined case, you could buy a Winchester single shot in black powder caliber for $15.00. No one wanted them. By rebarreling he put them back to work...
No shortage of whining in the world. And today we have the BR's and the Dashers and the PPCs and the Grendel and the SSKs and
... buy/read Mr. A's books and see how little is new except the names... Unfortunately this is how history has always been and I have given up on the idea that anything will change. "those who do not learn the lessons of history are doomed to repeat them..." luck to us allllllllll....