Or you could just listen to George Gardner, the owner of GA Precision who outlines why they developed the round.
As for the 7 & 300 PRC, the whole purpose is that they can easily use the long, high BC bullets with factory ammo in factory rifles. At no point could you buy factory 7 Rem Mag or 300 Win Mag loaded with the 180gr or 225gr class high BC bullets. I know this as I was hand loading the 212gr and 225gr ELD’s in my .300 WM since before the .300 PRC existed hence I haven’t felt the need to switch from the Win Mag.
For your other examples, the .22-250 sucks with AICS mags over 5 rounds and the majority are 12 or 14 twist drastically limiting any bullets over 60gr. The .260 had a standard 9 twist and was pretty much abandoned by Remington in less than 10 years. The .243 had an even worse 10 twist which pretty much eliminated the option of shooting any of the high BC bullets over 100gr.
Each of the Creedmoors and PRC’s has a tangible benefit over similar legacy cartridges to all but the most dedicated shooter willing to get a custom rifle made and handload ammo for it to get the same advantages with the older cartridges.
Considering the 145 ELD-X was specifically designed for the 10 twist of the 270 it would be more of an issue if it didn’t work.
I found out twist rates were a thing I should pay attention to when I loaded 105gr Berger Hybrids in a buddies .240 Weatherby, it would shoot 100gr soft points fantastic but that 105 hybrid would barely hit the broadside of a barn sideways at 50 yards.