Because it runs counter to your stated goal of convincing those reading the conversation to not take your interlocutor seriously. I really think if you're tempted to write a "I'm not going to argue with you" comment you should just not post at all: it damages your position less.
(This is a particular technique that seems to have been picked up because of gish gallops/sealioning where the goal is to just fill up the space and wear down any objections, but I think this cure is worse than the disease: explicitly refusing to engage, even if apparently justified by the expert view of the arguments you are rejecting, really only makes you look weaker, more unreasonable, and more out of touch to onlookers on the fence than walking away. I think it's a real hazard because it's emotionally satisfying to you and people who already agree with you while utterly corrosive to your credibility to anyone else)