It's reasonable to expect trust stores to be up to date but inevitably many won't be. A user contacted me with an issue a few days ago, turns out their server trust store (on Windows Server 2012) was significantly out of date. Obviously Windows normally keeps that sort of thing up to date but there will be administrators who suppress that either on purpose or accidentally.
I personally have (almost) zero sympathy for internet connected systems that are not appropriately updated or maintained, but I'm sure others are more forgiving 
- yes it can be someone else's fault that it's not up to date, but the buck stops with the administrator/owner.