Using weed is a recreation activity. Most recreation activities will have a negative impact on your life if they are not done in moderation. I don't think that is necessarily a problem and the fact that this generally isn't discussed in other contexts might indicate that people are subconsciously using this an excuse when their true objections to weed originate somewhere else.
Weed may be unique in allowing such a constant level of intoxication with relatively mild side-effects. I think that's how it gets people. Such addiction is not that uncommon and I've watched legalization in Canada with a mix of appreciation (I still do believe it should be legally available) and apprehension (I think we're about to see a small but significant % of an entire generation end up in the same place I've been for the last ~10 years.)
Cannabinoid Hyperemesis Syndrome
Definitely not the exact same kind of threat, since video games don't make you dumber like pot, but there is a real risk that a person substitutes accomplishing things in real life in favor of accomplishing things in a video game.
Your comment is kind of whataboutism. You're not really refuting what he's saying, just bringing up a separate topic