No one is saying that the .270 Winchester is not a great cartridge. It's the #2 Cartridge in terms of Ammunition Sales across the country, according to industry.
But the Short Mags have advantages... Shorter, lighter actions, more efficient generally means the cartridge is more naturally accurate, and I think it kicks less when comparing similar (399 Weatherby Vanguards) rifles, and it's faster with a flatter trajectory.
The .270 WSM and the .300 WSM are almost universal now. I've been out in the middle of no where in Wyoming and you can still find ammo for it. So that Availability arguement is kinda moot. The only downside to the WSM is that the ammo is a couple bucks a box more than the standard Winchester. Considering the advantages, I think the increased cost is just a good investment.