It is unlikely that the dark sector can eclipse the transparent sector in regards to global adoption, and right now there is no logical reason as to why Monero would drop the ball as the de facto leader in the dark sector.
I don't know, in the non-crypto world there is pretty good evidence for the underground economy, defined broadly (but in such a way that Bitcoin's full- or near-full transparency is neither useful nor desired), being
extremely large. Much of that at some point, along with portions of the entirely-above-ground economy, could conceivably move to Monero.
In recent literature on the informal economy, System D has become a shorthand name for the growing share of the world's economy which makes up the underground economy, which as of 2011 has a projected GDP of $10 trillion.