I get your points on Demar, and don't really disagree, but I do want to point out that the teams who have made deep playoff runs still have a guy who is pretty prolific at hitting midrange shots, especially in the playoff context. Middleton, Booker, Tatum, KD, Kyrie, Kawhi to name a few. Difference is, these guys can also knock down 3s at a high volume if they want, Demar hits one per week it seems like. Though an interesting comparison offensively lately is Jimmy Butler who has really toned down the amount of 3s he takes (and only makes 25% of 3s in his Miami tenure). Of course Butler brings so much more defensively than Demar.
All this to say, Demar ain't getting you anywhere as a 1st option for sure, but in the right team context, he would do just fine in the playoffs imo.