Imagine pushing 5kg worth of cocaine down a toilet while, most likely, incredibly strung out on said cocaine, with the SWAT team beating on your door. Couple that with the fact that they likely aren't stored in easy to open bags designed for pouring and what you're looking at is a bathroom full cocaine and a seriously clogged toilet. You've got to get rid of the plastic too, remember.
Maybe they bust you with less, but they'll still bust you.
If I am required to maintain documents in a civil case, and I destroy them, generally the other side is allowed to assume they were as bad as possible.
Does this apply in criminal trials? If there's evidence I was flushing drugs down the toilet (and such evidence would be there), can the state be allowed to assume I had a lot more of the drug than I really did? What are the policy implications?
Maybe they bust you with less, but they'll still bust you.