I look at it this way, his animal companion was banished. There is no immediate way to get back to their home plane and retrieve the animal. So, since it is his, and there is a bond, and the druid is 16th level, I'm okay with him taking a high level spell slot to get his animal companion back, for the duration of the spell, once every 24 hours. There is the danger of Dispel Magic, other protections against summoning, etc to consider, but overall, I don't see this as a game breaker.
Now the second it was abused.... then it wouldn't just plain wouldn't work.
Summoning named demons, devils, and gods is another matter altogether. I would think it very, very foolish to tamper with these beings and draw their attention. My players know better then to screw around with that type of thing. (I once had a devil torment a stupid PC at EVERY SINGLE moment it would be even a slight inconvenienced to him. This went down as an epic story still told today. Set the boundaries fellow DMs and you never will have any issues.)