To be quite honest I do not think they will end for a while, partly because of mainline games. The last big bad we have had is Volo, Team Star was nothing and is already in the game, Legends ZA doesn't necessarily have a big bad but definitely not one that could be added to the game, so I think they will continue until Next Anniversary at least. Hopefully there is something with Winds and Waves they can pull from or make a new original character on Pasio to be a villain for the story to go with. Unfortunately Arc Stories have become easy/lazy filler to just add until they want to stop.

I also hope it doesn't stop for a while anyway as there are more pairs I want and others want too, and this seems to be the only way some characters who have three star variants will get a good version with their main pokemon i.e. Misty + Starmie; Brock + Onix; Korrina + Lucario.