Having lived around the region for 3.5 years of my life...I can say this...the bulk of adults on the east side of DC smoke pot at least once a day. They’ve been doing it daily for years when it was illegal and this really doesn’t do much except make it a taxable substance (if you buy it via a licensed shop).
Based on the Colorado model, I’d say that the illegal street guys will continue their trade and at best....maybe half the pot in DC will be legal and taxed.
I don’t see this doing much except putting another couple million a year into the hands of the city revenue folks. Beyond that....the usage level will be the same.