No, it is not always safe to pour waste chemicals and solution down the sink with lots of water because it could be harmful to the environment. When you pour materials down the drain they either enter a septic system or a municipal sewer system. If the material enter a septic system, it goes into a tank buried underground where the solids settle and decompose. The rest of the wastewater is than released into a drain field where it goes into the soil. Sometimes the wastewater kills the good bacteria and can contaminate ground water or surface water.
A municipal sewer system the wastewater goes to a central sewage plant, where after treatment, it is then released into lakes, rivers and streams. Some hazardous household waste can pass through the system unchanged and pollute the waters. Also the wastewater may affect the plumbing or collect in the trap and release fumes through the drains. Most sewage facilities aren't capable of removing such toxic substances.
So, No, it is not always safe to pour chemicals and solutions down the sink even with a lot of water.
It is never safe to pour waste chemicals anywhere but a place where they can be properly and expertly disposed. While there may be a lot of water to dilute the solution, there is no telling what chemical compounds or substances are in the drain/sewage system, and if in an unfortunate event, the waste chemicals and preexisting ones do not mix well, then you may have a very dangerous situation.
Answer: Fossils are rare because most remains are consumed or destroyed soon after death. Even if bones are buried, they then must remain buried and be replaced with minerals. If an animal is frozen like the baby mammoth mentioned above, again the animal must remain undisturbed for many years before found.
