That's certainly something to be considered when outsourcing key parts of infrastructure yes.
I know the Pusher guys personally, so I'm confident they'll be around for the foreseeable future and I've integrated it into our code base in such a way that moving to someone else, or using our own server would be trivial.
It's something which needs to be evaluated on a case by case basis, I'm happy that all the components we outsource are both trivial to replace/insource and solid businesses who will be here for some time.
I know the Pusher guys personally, so I'm confident they'll be around for the foreseeable future and I've integrated it into our code base in such a way that moving to someone else, or using our own server would be trivial.
It's something which needs to be evaluated on a case by case basis, I'm happy that all the components we outsource are both trivial to replace/insource and solid businesses who will be here for some time.