Clickbait headline. Study found no decline in brain power.
Anecdotal: I've been ingesting about 4g of cannabis a day for 12 years now. If anything, I've seen my memory and cognitive abilities increase since I've begun.
From the article: "On average, the marijuana users who participated in the study consumed the drug three times per day."<p>^ Of course there's a brain decline if you smoke it 3 times a day. That's being perpetually high. If you drink alcohol heavily 3 times a day you'll also decline pretty rapidly.