Nah he won't fade in obscurity yet. It's not like K.Rool is dependant on Retro Studios to be considered at all. I mean, Retro Studios seem to go for new enemies each and every time in each game. However, PAON has succesfully used King K.Rool up to 3 times. And he's even playable in 2 of these appearances, and the other one he's a real legit boss battle.
PAON was involved with Brawl's development to for example. I'm betting they will return to make another Climber-esque game probably for 3DS. As we had King of Swing for GBA, and Jungle Climber for DS. Any DK-inspired racer would also likely be created by them. They used K.Rool in all games they made, and made him playable in 2. And a end boss in 2 as well. They are also the first who ever made him playable. If PAON is involved with Sakurai again, I see this as a big thing for King K.Rool's chances.
There's also Camelot wanting to make a DK64 sequel. And it's unlikely a direct DK64 sequel WOULDN'T have K.Rool in it.
The Vikings in Tropical Freeze aren't a treat to the Kremlings as well. It's much better they opted for new enemies instead of the Tikis again. Cause if we'd have Tikis yet again, we could argue they'd take away the roles of Kremlings as the new main enemies of DKC. Yet, there's no Tiki to be seen yet in Tropical Freeze. And better yet, this make the Kremlings and K.Rool the biggest treat to the Kongs yet, and still the main enemies of the DKC series.
K.Rool WILL return in a Donkey Kong game. His Smash appearance would surely help with this, but it isn't exactly needed. Though, I'd still be estatic if it happened of coarse.