That's not right. Refinforced concrete will suffer from environmental influences and have a durability from maybe 30 years. That means you have a cost overhead of 3-4% per year for the replacement value.

Water tanks certainly have a much worse life time, given that they will start lacking after a short time and you don't only have to repair the tanks but also provide liquid to refill.

Cycling stress like that is going to dramatically reduce the lifespan due to tress fractures. I think they would be lucky to get 10-20 years from the blocks.

Stress fracture are already a significant issue for concrete dams when you frequently vary the water levels, and this is much worse as different stacking is going to stress each block slightly differently every time. In theory they might be perfectly flat surfaces meeting evenly, but in practice that’s far from the case.

Wouldn’t you just grind them up and recycle them?

They can’t recycle it into blocks of the same material properties. It’s not worthless by any stretch, just a poor fit for such demanding applications.