Before you yell at me, hear me out: I know so many people who started smoking marijuana and their work abilities, their ability to hold difficult/in depth conversations, etc dwindled.

there's no doubt that for most people, cannabis does not make them sharp. it is a vice. it is not a performance enhancer. its a recreational intoxicant. it has downsides.

but at the end of the day what does it matter? there is no scientific consensus that it is significantly harmful to cognition over time. your conclusion that cannabis is "a negative thing" is just your opinion. i love weed, i know that it should be used in moderation like anything else, i'm stoned right now just like most days after work. and i work a tech job earning 6 figures. sure i'd be a bit sharper if i stopped smoking weed. i'd also be a bit sharper if i woke up at 5am every day, meditated for an hour, hit the gym religiously and gave up caffeine and alcohol entirely. cannabis gives me a very welcome escape, and i still pay my bills and contribute to my IRA and tell my parents i love them. not everything is about maximizing productivity or efficiency.

i think what's really happening here is that you've formed an opinion based on your real life experiences. which is totally normal. but something to remember--your experience is anecdotal. its a tiny, tiny sample of reality. and you also very likely formed a bias against cannabis users, leading you to perceive "cognitive decline" in your friends or the "many" people you know, who you observed before they tried cannabis, and after. and then you analyzed your observations and concluded that these people showed some sort of cognitive decline. if the pattern you're describing is real, why did you witness evidence of it, while the world's scientists studying cannabis didn't?