That's what I was thinking, but then the water would stagnate, go green, and be a potential source of mosquitos. So someone will want to change it.

The fix would obviously be to make a little bit of the water be drained away whenever a coffee was made, but then it would need refilling and you're back to a situation where you might as well just have 2 containers and disconnected from the water supply.

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.

The solutions that include an open air design for the bean hopper ignore the fact that oxygen is what causes beans to go stale. This is why coffee bean bags have a one-way valve built in: to let the CO2 out that is given off by the beans, and to prevent O2 from seeping back in.

And the solution to the problem is to install a barista.