How about the water container is transparent, and is actually used. Like a buffer containing water from the pipe. As it's used it get refilled.

Since the false water container on top isn't going to be used anyway, what about just filling it with something hostile to microbes, like dilute hydrogen peroxide? If you wanted to get really fancy, you could even add a couple lines marked "max" and "min" and add some plumbing to make the fluid level vary between them.

Apart from the problem of having potentially toxic substances as part of a beverage generation device, adding extra moving parts to simulate a non-moving part is really adding unnecessary complexity, effectively trying to fight against the universe (and the idiots that it produces).

Instead of a liquid, it could be filled with solid plastic, similar to the fake glasses of liquid you see in joke shops.

And perhaps a subtle label on the surface, for someone who notices that the container does not really have sloshing-around liquid in it: "No water required"

I might be wrong, but it's just water, I don't think it can go bad unless it's got bad things in it. And keeping it sealed would keep away the mosquitoes. If that ever even becomes an issue.