I got this idea long ago but I couldn't think of a way of using this idea. Then I thought it was obvious and that it was not implemented for some non obvious reason I didn't know.
A block of concrete is cheap and zero maintanance. And the machine typically is carried only twice in it's lifetime.
A water reservoir needs some way of filling, thus extra work for the buyer (typical ad: our washing machine doesn't need filling as those clunky reservoir ones!!!), maybe requires some maintenance. If you leave water in it for years it will rot, so probably you also need to put some disinfectants in.
So basically this idea is more environmentally sane, but more expensive. But there could be a market here, people care and are willing to spend more these days to save the env.
The soap water inlet can be a flexible tube that one can put inside the reservoir, then push a button and the exact amount of water is added (or some more and have it overflow to the drum).
> maybe requires some maintenance
Not if done correctly. And if it wasn't done correctly it seems easy to fix.
> so probably you also need to put some disinfectants in
