The 300 WSM has a fairly large following. It fits somewhere between the great 30-06 and the powerful 300 WM, but most of the fellows that shoot them say they kick like a 30-06 and hit like the 300 WM. I have seen some accurate rifles chambered in the 300 WSM. My bet is that this one is here to stay.
I had a 270 WSM and found reasons to not keep it. It did not seem to have any practical advantages over the standard 270 and did not feed very smoothly. The 338 RCM looks like a useful hunting cartridge but I doubt that it will ever be a big seller.
I would not be surprised if all of the WSSM cartridges drift into obscurity, if they haven't already. Some of the short fat cartridges do not offer anything except feeding difficulties, but I am sure that somebody somewhere loves them.