Prediction goes both ways. If your opponent is unwilling to fully utilize their Tapu Koko or whatever out of fear of a free A-Marowak switch in, then A-Marowak is doing its job. And if you predict that your opponent is trying to bait A-Marowak in so that they can hard switch to an unfavorable matchup, you can punish that play with hazard support or by simply calling their bluff. A-Marowak isn't going to be 6-0ing opposing teams for you, but it can provide support for teams that struggle with specific match-ups (in this case volt-turn or Electric Terrain teams) by forcing the opponent to make sub-optimal plays. And yeah, Marowak is slow as dirt, but its typing is obnoxious enough and its power / coverage are great enough that it is able to capitalize off of the positive match-ups it finds for itself. The possibility of A-Marowak switching in on your Xurkitree and forcing something on your team to eat a 210 base attack Shadow Bone or Flare Blitz is pretty frightening.Here is the thing. It just means there will be more hard switching if they see A-Marowak in the team builder. There are so many powerful water, ground, and dark pokemon in OU that it will be hard pressed to do much of anything effectively without having massive pressure put on it. It is so slow that it literally doesn't matter how strong it is.
Keep in mind I don't think that A-Marowak will be an OU or even UU staple, but I think that it has a clearly defined enough niche to at least be worth giving the benefit of the doubt to.