It is not economically worthwhile. You need several decades to get the money back. By then, something will have broken on the cistern. What will happen in the future is written in the stars.
And again, regarding your own quote: you don’t know whether the cistern will be economically viable. You say what will happen in the future is written in the stars. However, the signs are not good for individual squandering of drinking water for toilets and other uses. Presumably, the likelihood that resources will not continue to be so readily available is increasing. For reasons of impulse, I rashly had too many photovoltaic panels installed on my roof and, out of anger over a government decision here, I also impulsively bought a home battery. Two years ago it was really not economical, it cost me about 7000 euros. At today’s prices, the spontaneous purchase out of anger will have paid off in about 5-6 years. By accident, I did everything right. And that’s why I am envied. Back then, I was still able to buy at normal prices, which from today’s perspective were bargain prices, and I earn money through high feed-in. So this can also work out. In my opinion, the most correct approach is to install yourself to be as self-sufficient as possible.