It seems just way too easy to order drugs in 2015.
I've learned this since day #1: All good things eventually come to an end.
What changes/enforcement/policy can the government honestly make to put an end to all of this? Mailing narcotics through mail isn't anything new, but TOR can surely be compromised?
It depends on how you define it. If DNM's, in your mind, are exclusively the model we have today - TOR, onion services, with largely centralized marketplaces - I'm sure this will change at some point.
In my mind, the DNMs are just an extension of any black market. Black markets have existed ever since governments limited the buying abilities of the people (or put taxes on those goods/services). I have no doubt that there will be a digital black market for drugs for a long, long time. The protocols will change, the infrastructure will change, but the markets adapt. Money is the most powerful motivator we have and as long as money is to made, there will be innovation.
Encryption usually doesn't last forever. It serves it immediate purpose. Technology make previous encryption methodologies obsolete. PGP has been secure for quite a long time, mainly due to the increases in key lengths and thus requires more and more computing power. Can it go on forever like that? Probably not. Big advances in parallel or quantum computing could very well make our current system insecure.
But the one thing I'm sure of - the markets will adapt. I have no doubts. I have thousands of years of history that assure me of this.