I think your comment is misleading. What was "encouraged" during the Cape Town water crisis was not "small scale efforts". There were details like forbidding hand-washing in some places (replaced by hand sanitizers), but there were also important changes. For example, they introduced a punitive water price for big consumers.

Like the OP, I think one should analyze the problem before opting for the "obvious" solution. I did so with water a few years ago. IIRC, for individuals the biggest share is for showers, then toilet. These two account for two thirds of standard use.

If you encourage people to turn off the sink more often, you may gain 20%... of 5% of their average water needs, so 1% overall. If they take fewer showers, each with less water, they easily can halve their consumption, so a gain of more than 20% overall.

A water crisis is almost always a case of using slightly more water than is available, so you can usually address one by putting the burden on whichever consumers are least able to fight back, even if they are least responsible for the crisis.