Yeah, that's a good point. Aside from a few things like DKC, Mario RPG and Earthbound most major Nintendo SNES games are now on it. Unless Nintendo is going to figure out a way to get fancy with the Super Scope games or really feels the need to get Yoshi's Cookie or Vegas Stakes on the service then their contribution to the SNES library is going to come to an end pretty soon. What people are probably hoping for or complaining about not seeing are the third party games like the Mega Man X series, the Final Fantasies, Chrono Trigger, Mana, Earthworm Jim, Super Bomberman, Super Adventure Island and whatever else someone may have played at one time on the system.
Some games with Nintendo have rights issues like SimCity and Tetris Attack. Others with third parties also have rights issue like Disney games, Looney Tunes games, or Star Wars games as examples. As for other third party stuff, Capcom recently released the Mega Man X collections. Doubt they will now put those games up on the service. Square has done a Mana collection. Don't see Square putting FF4 or FF6 on the service. Hudson Soft is dead. Are some companies going to bother with their properties like Clayfighter or Zombies Ate My Neighbors?
I could see Natsume maybe providing some titles like Harvest Moon, Lufia or Pocky and Rocky. And there might be a few other deep cut surprises by third parties like Pac-Man 2: The New Adventures. But overall, it is probably going to get harder and harder to add things to it. Moreover, how many games do these services need? Sure, it would be great to see 100 SNES games but if there are 40 overall, that's still a fair chunk of gaming time one could spend on it. More likely, we'd see other systems added instead as Nintendo moves on to other parts of their past library to try and utilize instead.
Scrolling through a list of SNES games, my goodness there were a lot of great licensed games that just probably are getting re-released.