Besides Brass quality and availability, I think the 243 wssm didn't get much traction because the good ol 243win already did what the 243wssm was supposed to do, Without most of the downsides.
I agree a 243 win or 243 AI will do everything the WSSM will do but who doesn't want to be different.
I'm lumping both of these together just for the sake of space and time.
I quite frankly don't experience 'downsides' to running any of these cartridges. They perform above most every day expectations and when properly handled in the platform of choice, perform like any other cartridge.
It is often discussed that just because these cartridges have similar capacities to the venerable old .243 Win. that the performance will be exactly the same. I have not found that to be true even when attempting to keep the comparisons similar. I can achieve 3,300 fps with the WSSM case while the .243 Win. lags behind running all out trying to achieve 3,100 fps. This with a 105 grain Berger Hybrid, and yes I even throated both chambers to attempt to take advantage of the 'extra powder space'...
We're all aware of the vagaries of trying such comparisons because of the number of variables present in the systems. Even a barrel can be faster or slower than another barrel straight from the factory. Powder lots can be significantly different from the manufacturer. But when I run these comparisons consistently over time, the range holds true.
The internet is guilty of many things, two of which are aggravated assault and outright lies being perpetuated by the speed of digital data.
Find just one discussion about the 6.5 Creedmoor and you'll read what I mean. The divisiveness is unacceptable since we're all on the same side.
Now the bigger point is that there should be room in the shooting world for cartridges such as the WSSMs and others since they are decidedly different from the .308 Win. based cases. All of these comparisons thrown about on the internet serve only to attempt to disparage the newer cartridges. Yes, you know the examples; 6.5 Creedmoor/.260 Win., 6mm Creedmoor/.243 Win., 6.5 PRC/6.5x284 Norma, et al.
I like having choices.

Bring on the newest cartridge, let's appreciate the fine tuning achieved by the manufacturers who have invested in bringing something better to the table, even it's only just a tiny improvement.
Yes, I really do like to have something different and the WSSMs usually accomplish this when demonstrated to other shooters.

