For "most", anything over 48kg will not be sustainable or the smart programming choice over the long haul.
CSCS Guy
As a CSCS guy, you are familiar with this stuff.
With that in mind, let me add some additional information.
The information presented not necessarily for you but other who might have question on this.
48 kg
What makes a 48 kg the magic number?
As Kettlebellelephant noted, the load is somewhat dependent on the body weight of the individual. The other factor is their Maximum Strength.
My previous posts on this have provide some guidelines on the Kettlebell load based on body weight required for Power Development.
Sustainable
I am not quite sure how you are defining this.
With that said, all program follow some type of Periodization Training Plan; a sine curve of starting off with a light load. Progressively increasing the load during a training cycle to a final week of intensive training, then dropping the load and starting over with something light and easy.
That because sustaining Heavy Kettlebell Swings is as you stated, not sustainable; no heavy intensive training is.
Power Development
Power is developed within specific training ranges/percentages.
Most individual interested in developing Power are not performing Swings with a Kettlebell that is heavy to optimize it.
That due to the lack of knowledge in regard to it. Secondly, most individual don't have a Kettlebell heavy enough.
Go or Show
Some individual probably do perform a Heavy Kettlebell Swing for show. However, that doesn't mean everyone does.
My Heavy Swings, as well as Al's and even Anna's are more about "Go than Show"; Power Development.
Heavy Swing Over The Long Haul
They are sustainable over the long haul, providing a well written/performed Periodization Training Cycle is adhered to.